…0179)

* feat(tidb): add AsPingCapAST bridge on OmniAST (Phase 1.5 §1.5.2)

Mirror of mysql/OmniAST.AsANTLRAST for the Phase 1.5 advisor migration.
Required prerequisite for the eventual dispatcher flip (§1.5.N+1):
once the registered ParseStatementsFunc is flipped to omni and
*OmniAST values reach the ~50 un-migrated tidb advisors at any point
in the migration, those advisors keep working because GetTiDBAST
falls through to AsPingCapAST() automatically.

Implementation mirrors mysql/omni.go:39-55:
  - Lazy parse: native pingcap parse runs once on first AsPingCapAST
    call per OmniAST instance.
  - Cached result: pingcapParsed bool flag distinguishes "not yet
    parsed" from "parsed and got nil"; cache hits skip the work.
  - Negative caching: pingcap parse failure also short-circuits future
    calls so unparseable-by-pingcap SQL doesn't re-parse per advisor.

backend/plugin/parser/tidb/ast.go:
  - Adds PingCapASTProvider interface (analog of base.AntlrASTProvider).
  - GetTiDBAST extended to check for the provider, mirroring
    base.GetANTLRAST's pattern.

backend/plugin/parser/tidb/omni.go:
  - OmniAST gains pingcapAST + pingcapParsed unexported fields.
  - AsPingCapAST method implements the provider contract.
  - Godoc updated: removes the "neither fallback" rationale (correct
    for PR #20107's scope but inverted by the migration plan), points
    at plan §1.5.0 invariant #4 + bridge persistence rule for context.

Tests:
  - TestAsPingCapASTReturnsNativeAST: bridge contract for valid SQL.
  - TestAsPingCapASTCachesAcrossCalls: a.Same() pointer equality on
    repeat calls proves cache hit, not fresh re-parse.
  - TestAsPingCapASTCachesNegativeResult: parse failure cached too.
  - TestGetTiDBASTFallsBackToProvider: end-to-end bridge through
    GetTiDBAST — the path un-migrated advisors will use.

Bridge persistence: the bridge stays in the codebase indefinitely
post-Phase-1.5. The one Class III advisor (dml_dry_run, blocked on
omni's NonTransactionalDMLStmt grammar — Phase 2 §Tier 4g) requires
this fallback. Bridge cleanup is gated on dml_dry_run unblocking, not
on Phase 1.5 completion.

Validation:
  go test -count=1 ./backend/plugin/parser/tidb/...    # green (incl. 4 new bridge tests)
  go test -count=1 ./backend/plugin/advisor/tidb/...   # green
  go test -count=1 ./backend/plugin/advisor/mysql/...  # green (no cross-engine regression)
  go build ...                                         # green
  golangci-lint run ...                                # 0 issues

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(tidb): bridge line-tracking + symmetric logging + scoped godoc

PR review caught three issues. All addressed before merge.

#3 (ship-blocker) — Line-tracking regression. The original bridge called
bare ParseTiDB and skipped the SetOriginTextPosition + per-column line
work that the canonical pre-flip path (ParseTiDBForSyntaxCheck) applies
at tidb.go:64-75. 49 of 51 tidb advisors call node.OriginTextPosition()
for advice line numbers; without the fix, post-flip un-migrated
advisors silently regress to line 1 for every statement.

Fix: extract applyTiDBLineTracking(node, baseLine, originalText) helper
from ParseTiDBForSyntaxCheck. Both the canonical path and the bridge
call it, so consumers reading OriginTextPosition() see consistent
values across both paths. Helper returns the 1-based actualStartLine
for *AST.StartPosition use.

Regression test: TestAsPingCapASTLineTrackingMatchesCanonical parses a
two-statement input via both paths and asserts the second statement's
node.OriginTextPosition() matches (both must report 2, not 1).

#1 — Godoc overpromise. Original wording claimed "review continuity
from both directions" but the bridge only handles one direction —
omni-success → pingcap-fallback. The opposite (omni rejects at
dispatcher level so no *OmniAST exists) is the dispatcher-fallback
contract, not the bridge's job. Godoc now narrows the claim and
forward-references plan §1.5.0 invariant #5 (the dispatcher-fallback
invariant being added in a follow-up plan amendment).

#2 — Logging asymmetry. Pilot side soft-fails with slog.Debug + log.
Bridge silently returned (nil, false). Now logs symmetrically on each
failure mode (parse error, zero nodes, line-tracking error). Negative
cache means a single Debug per OmniAST instance, not per call.

#4 — Strict ParseTiDB note. Godoc now explains why the bridge uses
strict ParseTiDB (no error-recovery) instead of mysql's lenient ANTLR
parse: pingcap has no equivalent error-recovery mode, and the bridge
operates on omni-validated SQL where strict parsing is correct.

#5 — Multi-statement assumption made explicit. nodes[0] used; nodes[1:]
logged as a contract violation instead of silently dropped.

Validation:
  go test -count=1 ./backend/plugin/parser/tidb/...   # green incl. new regression test
  go test -count=1 ./backend/plugin/advisor/tidb/...  # green
  go test -count=1 ./backend/plugin/advisor/mysql/... # green (cross-engine regression)
  go build ...                                        # green
  golangci-lint run ...                               # 0 issues

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(tidb): correct godoc reference to plan invariant #8

Numbering drift caught in review: the godoc cited "§1.5.0 invariant #5"
but the landed plan invariant is #8 (the dispatcher-fallback contract).
The §5 numbering came from an earlier draft of the plan amendment;
final landed numbering is #8 because three additional invariants (#5
no-mutation, #6 recipe-default, #7 shape-audit) had already accreted.

One-line fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(advisor/tidb): soft-fail getTiDBNodes on bridge miss

Codex P1 catch on PR #20179.

Without this fix, post-§1.5.N+1 dispatcher flip:
  ParsedStatement.AST = *OmniAST → un-migrated advisor calls
  getTiDBNodes → tidbparser.GetTiDBAST(stmt.AST) →
  AsPingCapAST fails (pingcap rejects what omni accepted) →
  GetTiDBAST returns (nil, false) →
  getTiDBNodes returns 'AST type mismatch: expected TiDB parser result' →
  advisor.Check propagates as 'Rule check failed' →
  every un-migrated advisor breaks for that review

That breaks the migration continuity the bridge is meant to preserve.
Phase 1.5 invariant #2 says soft-fail (skip statement, review
continues), not hard-fail.

Fix: when GetTiDBAST returns false AND stmt.AST is a
PingCapASTProvider (i.e. an *OmniAST that just routed through the
bridge and missed), skip the statement instead of returning an error.
The bridge has already logged the parse failure at debug level.

True engine-mismatch (stmt.AST is some unrelated type, neither
*tidb.AST nor a PingCapASTProvider) is still surfaced as an error —
that's a programmer error in registration, not a soft-fail case.

Regression test:
  TestGetTiDBNodesSoftFailsOnBridgeMiss feeds a context with two
  ParsedStatements: one *OmniAST whose AsPingCapAST will fail
  ('SELECT FROM WHERE;') bracketed by one that succeeds ('SELECT 1').
  Expects no error and len(got) == 1 (the successful one).

Sanity-checked: GetTiDBAST returns false on a pingcap-invalid OmniAST,
confirming the pre-fix path would have errored.

Validation:
  go test -count=1 ./backend/plugin/advisor/tidb/...   # green incl. new test
  go test -count=1 ./backend/plugin/advisor/mysql/...  # green
  go test -count=1 ./backend/plugin/parser/tidb/...    # green
  golangci-lint run ...                                # 0 issues

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>