fix(sse): share hardened replay cancel with JSON-to-SSE path

[maxmad64bis]

⚠️ base-red inherited: #14547

Summary

The JSON-to-SSE replay path awaits the provider cancel promise when it discards a replayed stream, and that promise may never settle, so the cancel can hang. This change reuses the bounded fire-and-forget replay cancel already used by the readiness path, so discarding a replayed stream always returns.

Related Issues

Related to #13171 (reader handoff this path preserves) and #12465 (one-chunk-per-demand replay semantics this change generalizes to the second site).

Validation

• Change type: provider
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/json-body-replay-cancel-bounded.test.ts (new, 2 cases: SSE-preserve path cancel returns under 500 ms with a never-settling provider cancel — RED stayed pending then GREEN; shared-symbol path with prefix order preserved).
• Neighbors re-run one by one, all green: tests/unit/chatcore-json-body-to-sse.test.ts (6/6), tests/unit/jsonbody-sniff-reader-leak-13169.test.ts (2/2, happy-path handoff guard stays green), tests/unit/stream-readiness.test.ts (21/21), tests/unit/stream-readiness-policy.test.ts (23/23).

Coverage Notes

• New cancel path covered by the new test plus the four neighbor suites above; no coverage gate movement expected.

Reviewer Notes

• open-sse/handlers/chatCore/jsonBodyToSse.ts shrinks by 28 net lines (local copy removed, one import added); open-sse/utils/streamReadiness.ts gains one export keyword; no rebaseline.
• Mutation drift (--strict) reports 5 missing tap.testFiles entries, all on modules outside this diff; none on the touched modules, so it's not registered in this PR (pre-existing drift on the base).
• CI failures are inherited from the base, not from this diff: Docs Gates fails on stale migration counts in README.md/AGENTS.md/llm.txt ("183 migrations" at the base vs 186 in code, untouched since mb — diff $mb empty, same failure on same-base PRs refactor(stream-recovery): drop unreachable empty-continuation block in tryContinue #14789/fix(sse): return over-cap bounded read without awaiting tee clone cancel #14791/fix(api): thread correlationId into chat completions early keepalive #14792/fix(sse): correlate mid-stream continuation logs with requesting call #14793) and the 4 Unit Tests shards plus Fast Quality Gates fail on 21 tests whose normalized list matches the same-base refactor(stream-recovery): drop unreachable empty-continuation block in tryContinue #14789 list except one usage-pending-sweep case that names no file of this PR (test file untouched since mb — diff $mb empty on it); API Route Typecheck, Merge integrity, ESLint, Vitest pass.