fix(openai): complete Codex state and compact probing

[lyy0709]

Summary

Follow-up to #5668, limited to the Codex state-machine, remote-compaction capability probe, and beta/default upgrade gaps that remained after that PR.

• bind each exact opaque x-codex-turn-state value to the OAuth account that actually delivered it, persist only a scoped digest across instances, and strip only known cross-account echoes
• record provenance only after the downstream header/body write succeeds; cover native and passthrough, streaming and non-streaming, including already-committed compact keepalives
• keep OpenAI API-key traffic transparent: relay the header but never create or enforce OAuth provenance
• make the compact probe follow Codex RemoteCompactionV2 collection semantics: require response.completed, count only response.output_item.done, require exactly one compaction item, permit unrelated done items, and ignore bytes after the first terminal
• preserve the official compaction_summary serde alias, while removing added-only fallback and compaction deduplication that could hide malformed streams
• add tri-state, versioned, expiring probe snapshots; bounded response reads; cancellation-detached persistence; monotonic database updates; and a migration that invalidates snapshots created by the old permissive probe
• normalize beta-feature headers case-insensitively and preserve native-v2/session defaults without changing API-key Responses routing

Official Codex reference audited at 49db349ffdd888f5e3c91abf9b7519d8631e6e9a:

• RemoteCompactionV2 collector
• RemoteCompactionV2 request construction
• Compaction response item aliases

Regression coverage

New and expanded tests cover:

• exact opaque state values, same-value recommit, API-key/session isolation, cross-instance lookup, cancellation, write failures, keepalive-committed headers, and all four native/passthrough stream modes
• LF/CRLF/lone-CR SSE, BOM, comments, multi-line data, no-colon fields, EOF dispatch, [DONE], terminal ordering, added-only output, duplicate compaction items, aliases, incomplete/truncated/oversized/read-error bodies, and 2xx/error status matrices
• supported/unsupported/unknown transitions, protocol version/TTL/skew boundaries, concurrent 24-writer monotonic convergence, unknown diagnostics, duplicate-account cleanup, and migration behavior/idempotence
• beta token casing, duplicate removal, override preservation, native-v2 forcing, OAuth defaults, API-key transparency, and probe identity parity

Verification

• cd backend && go test ./... -count=1
• cd backend && go test -tags=unit ./... -count=1
• cd backend && go test -tags=integration ./... -count=1
• targeted go test -race for the changed service/repository paths
• cd backend && golangci-lint run ./...
• make test
• pnpm --dir frontend run test:run (223 files, 1547 tests)
• pnpm --dir frontend run test:coverage (223 files, 1547 tests)
• make build

No frontend, dependency, lockfile, or version metadata changes are included.

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