fix(sse): return over-cap bounded read without awaiting tee clone cancel

[maxmad64bis]

⚠️ base-red inherited: #14547

Summary

A buffered turn that exceeds the size cap hangs the request forever: the over-cap branch awaits a tee clone cancel that only settles once the original is read, but the original is piped to the client below. This change returns the verdict without awaiting the cancel, matching the neighboring idle branch.

Related Issues

No linked issue — no upstream report covers the over-cap tee cancel hang (searched tee clone cancel, reader.cancel, emptyTurnRetry over: no open match at a41ded27cc).

Validation

• Change type: sse
• 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/flush-empty-retry.test.ts: new regression case with a stream that exceeds the cap and never closes, asserting the outcome returns without waiting.

Coverage Notes

• The change touches open-sse/utils/emptyTurnRetry.ts; the new regression test covers the over-cap branch, and the neighboring suites stay green.

Reviewer Notes

• The 4 failures in the file are identical on the untouched base (base run: 18 pass / 5 fail incl. the new case; head run: 19 pass / 4 fail) — pre-existing, non-blocking, not listed in 🔴 Release branch not green: release/v3.8.51 #14547.