fix(combo): surface context-overflow before compression so oversized requests fail fast with a clear error (#10225)

[diegosouzapw]

Closes #10225

Combo requests whose combined context exceeds the model/context budget previously reached compression and yielded a late, generic failure instead of a clear context-overflow rejection.

Root cause: the context-overflow check ran after (or was bypassed by) the compression path, so an oversized request's failure was ambiguous and late.

Fix: check knownContextOverflow/context budget during dispatchPrelude and targetResolution, surfaced through combo.ts`` + chat.ts` as an early, explicit error before compression runs.

Regression test (RED→GREEN): tests/unit/combo-context-overflow-compression-probe.test.ts — 3 assertions GREEN (oversized request rejected explicitly pre-compression).

Gates: typecheck exit 0 · eslint exit 0 · file-size OK · complexity OK · cognitive OK · changelog-integrity OK · combo-context-overflow probe 3/3 GREEN.

⚠️ base-red inherited: #9985

[diegosouzapw]

Request changes. The new preflight deferral is not aligned with chatCore's actual compression eligibility: for a codex Responses target, shouldUseNativeCodexPassthrough() is true, and chatCore sets compressionExcluded from nativeCodexPassthrough, disabling both prompt compression and reactive compaction. However, resolveComboContextOverflowDeferral() returns defer=true based only on the global switch and API-key opt-out, while getKnownContextOverflow() checks only configured exclusions. A raw-over-limit codex request therefore skips the old early 400, reaches chatCore without compression, and is rejected by the final raw context gate rather than being compacted. The added probe passes because handleSingleModel is mocked to return 200 and never enters chatCore. Focused evidence: the PR probe passes 3/3; a head probe prints {"native":true,"deferralCondition":true}. Please make deferral target-aware and add a real chatCore regression for both compressed-success and compressed-still-too-large cases. The base-red #9985 remains inherited and is not attributed to this PR.