fix(streaming): resume truncated Responses streams mid-stream

[maxmad64bis]

⚠️ base-red inherited: #14547

Summary

Mid-stream continuation only understands chat-completions events behind a translated chat request: on the Responses upstream path the scan stays shut, the continuation body requires messages, and a short complete turn has no recognized terminal marker, so an enabled setting silently does nothing there. The recovery layer now scans Responses events (text and reasoning deltas, function-call signals gated on function_call items, completed/failed/incomplete terminals), replays received text as an assistant input item, and stitches the suffix plus a minimal response.completed snapshot in the upstream envelope so the downstream translator still decides stop versus tool_calls. Tool-call, overlap, and reasoning-only guards are unchanged; chat behavior is untouched.

Related Issues

Related to #4147 (origin: chat-only mid-stream continuation this PR extends to Responses)
Related to #11151 (prior: reasoning-only resume whose text-only guard this PR reuses)
Related to #11902 (orthogonal: client-side retryable signal; this PR is the server-side resume)

Validation

• Change type: routing
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward (rev-list HEAD..upstream/release/v3.8.51 = 0, unit 24/24 + neighbors 88/88 and touched-file ESLint clean rerun after push @ 0c4724fd)
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/stream-continuation-responses.test.ts (new, 24 tests: Responses scan, terminal markers, continuation bodies, short-complete never resumed, cut-resume stitching, tool-call OFF/ON, translator traversal stop/tool_calls/idempotent; RED 7/17 on the base, GREEN 24/24 here)
• Replayed neighbors: stream-continuation, stream-continuation-wiring, stream-recovery-toolcall, stream-recovery, stream-recovery-trace-logging, chatcore-stream-recovery-log-wiring — 88/88 pass

Coverage Notes

N/A — no coverage gate moved; new branches in streamRecovery.ts are covered by the new test (scan, continuation body, stitching per format).

Reviewer Notes

• New probes, no behavior change by default: an all-chat stream never sets the Responses flag, and chat stitching is the pre-existing branch.
• A short complete Responses turn ending in response.completed is never re-requested (flush, no retry); failed/incomplete end the turn without resume.
• Update 2026-09-23 (post-ready CI, head 766ffdb): Fast Quality Gates flagged one owned-file regression (streamRecovery.ts complexity 5→8, cognitive 3→4, new-code mode), fixed by splitting into smaller helpers with identical semantics (amended, lease 0c4724fd → 766ffdb); local gate now complexityNewCode=0 / cognitiveComplexityNewCode=-1, unit 24/24 + neighbors 88/88 replayed. Remaining reds qualified as inherited (failures cite files outside this PR, unchanged since the merge-base, base-red issue 🔴 Release branch not green: release/v3.8.51 #14547, normalized failure lists identical to same-base feat(sse): distinguish content-stall from transport failure in stream error codes #14582): API Route Typecheck 4 errors (auggie.ts TS2769/TS18047, projectCombo.ts TS2459/TS2724, no PR file cited, comm -23 empty vs feat(sse): distinguish content-stall from transport failure in stream error codes #14582); typecheck:core single error in cliproxyAccountHealth.ts identical on the clean base checkout (out of scope); full npm run lint red on the base (🔴 Release branch not green: release/v3.8.51 #14547); check:docs-all DEEP_HEALTH_CHECK_ENABLED present on the untouched base (health/route.ts:46) outside the PR, existing flag of the resumed test kept as is.

[diegosouzapw]

Substantial and well-tested extension — the fold/sink refactor of the scanner is a clean
response to the complexity-gate feedback, and the RED-7/17→GREEN-24/24 test evidence plus the
88/88 neighbour replay is exactly what this kind of change needs. Ran the new suite (24/24) on
a tree with the rest of this session's stall/buffered-turn batch merged in — no conflicts, no
shared files with #14691 despite the topical overlap. No blocking items.