fix(sse): re-enable prompt compression for native Codex passthrough

[thomasmaerz]

Summary

Native Codex passthrough (POST /v1/responses, provider=codex, e.g. opencode → omniroute) silently stopped producing compression savings: 100% of codex rows in compression_analytics became skip_reason='excluded', mode='off', tokens_saved=0 while call_logs volume continued.

Root cause: Commit 32f83407 (2026-08-11, #8933) added nativeCodexPassthrough || to the prompt-compression exclusion gate. Carried onto release line via 65e81158a (v3.8.50, #11088 sync).

Fix (prompt-only scope)

open-sse/handlers/chatCore.ts: compressionExcluded no longer includes nativeCodexPassthrough; prompt compression now depends only on the operator exclusions list (#8034). Native contexts compress via the existing adaptBodyForCompression() Responses→messages adapter with codex tool-output eligibility guards. Operators who want byte-identical passthrough can add codex/* to exclusions.

Deliberately NOT changed (reactive + combo deferral)

• Reactive proactive/last-resort compaction (!nativeCodexPassthrough gates in chatCore.ts) still bypass native passthrough. These passes rewrite history (trim_tools/purify_history) and are the overflow safety net — re-enabling them for stateful Responses contexts needs its own validation.
• Combo overflow fail-fast (dispatchPrelude/knownContextOverflow, #10225 / #10503) still treats native-codex pools as incompressible, so oversized requests keep failing fast locally instead of sailing upstream uncompressed.
• Net effect: normal-sized codex requests compress again (analytics return); oversized codex requests still fail fast without attempting compression. That asymmetry is intentional and pinned by the updated #10503 comments.

Regression Analysis & Verification

See issue comment: #12793 (comment)

Timeline:

• Aug 6 ([codex] Honor disabled compression for reactive compaction #9200): Added reactiveContextCompactionEnabled to honor global compression "off" for reactive compaction
• Aug 11 (Fix custom tool output pairing during context compression #8933): REGRESSION INTRODUCED - added nativeCodexPassthrough || to compressionExcluded, unconditionally disabling ALL compression for Codex /v1/responses
• Aug 23 (65e8115): Sync commit carried both to release line → v3.8.50+ broken

Why #9200 didn't prevent this: #8933 came AFTER #9200 and added a NEW unconditional bypass that overrode the global switch #9200 introduced.

What #8933 actually fixed (verified still intact with my PR):

1. Executor repair (codex.ts): repairMissingCodexToolCallOutputs - ensures function_call_output items exist for every function_call ✅ (43/43 tests pass)
2. Compression bodyAdapter (bodyAdapter.ts): Enhanced to properly track/restore custom_tool_call ↔ custom_tool_call_output pairing during compression ✅ (11/11 tests pass)
3. Combo overflow (clientManagedResponsesContext flag): Allows native Codex clients to self-manage context overflow ✅ (17/17 tests pass, including "native Responses context reaches an all-Codex target beyond its catalog hint")

#9200 fix verified intact: Global compression "off" still gates reactive/last-resort compaction ✅ (1/1 test passes)

Validation

• New regression guard tests/unit/codex-prompt-compression-passthrough.test.ts (3/3 pass; first assertion fails on pre-fix source).
• Existing suites green: compression-exclusions (8/8), chatcore-request-format (9/9), combo-context-overflow-compression-probe (5/5 substantive incl. end-to-end chatCore compression dispatch; file has pre-existing teardown DB handle hang unrelated to this change), executor-codex (43/43), body-adapter (11/11), combo-context-window-filter (17/17), reactive-context-compaction-policy (1/1).

Related

Closes #12793