Summary

Reject Messages streams that interleave content blocks with terminal message deltas. A message_delta while a block is open, or a new block after a message delta, now produces a fixed api_error before tool dispatch or successful completion. Previously, these invalid sequences could still execute a search.

This follows the Messages streaming event flow: content blocks precede one or more message deltas. Repeated cumulative usage snapshots remain valid. The existing ingestion path and per-round state own both checks.

Part of #313 and Enterprise Readiness #316; follow-up to #396. Full schema validation and retained-resource limits remain open.

Test Plan

• Regression failed before the fix with an unexpected search request. Covers a message delta before content and during an open block in both recorded rounds.
• Repeated-terminal-snapshot control preserves one public lifecycle and unchanged cumulative usage. Existing open-block test now omits message deltas to continue exercising the message-stop gate directly.
• Core library on current main: 1,068 passed, 5 ignored. Messages stream integration: 28 passed, including the merged payload/identifier and extension-delivery regressions from fix: validate Messages block payloads before assembly #411.
• Workspace all-target Clippy with warnings denied, formatting, diff check and all-files pre-commit checks passed.
• Deterministic local servers and existing recordings; no live inference.

Reconciled with main after #411 merged. Retains all merged validation and extension-delivery assertions; the terminal-order guards are unchanged.