Pin query plan settings that affect EXPLAIN output structure:
- query_plan_filter_push_down=1: controls filter push-down below
  aggregation/sort/distinct steps
- query_plan_merge_expressions=1: controls Expression step merging
- query_plan_execute_functions_after_sorting=1: controls function
  lifting after sorting
- query_plan_split_filter=1: controls filter expression splitting

CI randomizes these settings (PR ClickHouse#100874 adds them to
SettingsRandomizer), which changes step ordering and breaks the
grep-based EXPLAIN assertions. Each test now pins exactly the settings
that control the optimizations it verifies.

Affects: 01655_plan_optimizations, 01655_plan_optimizations_merge_filters,
01655_plan_optimizations_optimize_read_in_window_order
(549 total failures across 25 PRs in 30 days)

Pin query_plan_merge_expressions=1 and query_plan_merge_filter_into_join_condition=1
to prevent CI settings randomization from changing the EXPLAIN output structure.

The test verifies that WHERE filters get merged into JOIN conditions. When either
setting is randomized to False, the filter stays as a separate step and Expression
steps are not merged, causing the EXPLAIN output to differ from the reference.

53 failures across 7 PRs in 30 days, predominantly from PR ClickHouse#100874 which adds
these settings to the randomizer.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Pin `query_plan_remove_unused_columns = 1` in the EXPLAIN query to
stabilize skip index EXPLAIN output.

When `query_plan_remove_unused_columns` is randomized to False (by PR
ClickHouse#100874's settings randomizer), unused columns remain in the query plan,
shifting text index INPUT node positions from 0 to 1+ and removing the
"Actions:" prefix from the output. This causes the reference file
comparison to fail.

The fix pins the setting only in the EXPLAIN query (query-level SETTINGS),
so it does not affect the actual query execution or data correctness checks.
The test still verifies that text indexes work correctly in PREWHERE with
both skip_indexes=0 and skip_indexes=1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>