fix(sse): reject a low-overlap stream-recovery continuation instead of concatenating it raw

[maxmad64bis]

Summary

The mid-stream continuation (Fase 4.4) re-requests with the already-emitted text as an
assistant prefill and expects the model to continue verbatim. trimContinuationOverlap
correctly reports "little/no overlap found", but the caller in tryContinue treated any
non-empty result as a successful stitch and appended it straight onto the emitted text
with no separator — producing a message cut in mid-word whenever the model ignored the
prefill and restarted on a fresh sentence instead of continuing.

A zero-overlap-only check is not enough: a model that
continues cleanly with zero echoed tokens is a legitimate outcome, indistinguishable
from a silent restart using string data alone. This PR uses a documented threshold
(STREAM_RECOVERY.MIN_CONTINUATION_OVERLAP_CHARS = 8) instead of an exact-zero check,
as a measured trade-off — see the constant's doc comment in open-sse/config/constants.ts
for the explicit false-positive/false-negative analysis. trimContinuationOverlap itself
is untouched — its "no overlap → return unchanged" contract is correct at the utility
level; the bug was purely in how the caller interpreted that result.

Related Issues

• Related to fix(sse): resume stream recovery after a completed tool call #11109 (adjacent file, distinct subject — kept separate per one-fix-one-PR).

Validation

• Change type: sse
• Focused tests + category gates: node --import tsx/esm --test tests/unit/stream-continuation.test.ts tests/unit/stream-continuation-wiring.test.ts tests/unit/stream-recovery.test.ts → 34/34 PASS
• Full focused matrix (integration + resilience) → 46/46 PASS
• npx eslint open-sse/services/streamRecovery.ts open-sse/config/constants.ts tests/unit/stream-continuation-wiring.test.ts → 0 errors
• Reconciled with release/v3.8.50 tip (cut from origin/release/v3.8.50)
• Production-code changes include new/updated tests in this PR
• ⚠️ base-red inherited: 🔴 Release branch not green: release/v3.8.50 #9985 (release/v3.8.50 not green — known, not introduced here)

Tests Added Or Updated

• tests/unit/stream-continuation-wiring.test.ts — zero-overlap and below-threshold
  restarts are rejected and never concatenated raw; an overlap at/above the threshold is
  still stitched correctly (regression guard); the pre-existing "silent post-commit
  truncation" test's fixture was adjusted to an above-threshold partial overlap
  ("there world", 11/18) to make the trade-off explicit rather than silently masked.

Coverage Notes

• Change touches open-sse/services/streamRecovery.ts + open-sse/config/constants.ts.
  Covered by the unit tests above.
• The pre-existing "recovers a post-commit transport error" fixture was also moved to an
  above-threshold overlap ("Partial answer done.", 8/8) so it still exercises the happy
  path under the new gate.

Reviewer Notes

• overlapChars is derived as scan.text.length - overlapResult.length (== matched k);
  correct today, binds to trimContinuationOverlap semantics. Documented as a heuristic,
  not a solved distinction.
• Post-merge follow-up: instrument the false-positive rate of
  isSuspectedRestart; threshold 8 is uncalibrated.