fix(responses): refuse input beyond the advertised context window (#1128)

[HoshimiRox1]

Summary

Supersedes #1167 after rebasing the contributor branch onto current dev; GitHub does not allow reopening a closed PR whose head history was rewritten.

• Reject parsed /v1/responses text input when the shared model-aware token estimate exceeds the final routed model's advertised context window.
• Run the guard after previous_response_id expansion and final route selection, but before authentication, adapter construction, or upstream I/O.
• Contain abnormal replay expansion observed in production: a roughly 400k-token conversation became a 1.6M-token request, ballooned Bun RSS on Windows, and native-crashed the proxy. The guard returns a clean 413 without calling upstream.
• This PR is intentionally limited to the independent input guard. The earlier DeepSeek compaction reframe was removed after upstream dev commit 0b8e608 restored live Responses streaming. Refs [Bug] DeepSeek V4 Flash remote auto-compaction needs v2.11+ reproduction #1128.

Verification

• bun test tests/responses-input-guard.test.ts — 2 pass, 0 fail.
• bun run typecheck — pass.
• bun run privacy:scan — pass.
• git diff --check — pass.
• Full bun run test was attempted on Windows: unrelated codex-journal restoration tests remained red in isolation (8 failures), and the full run ended in a Bun 1.3.14 index-out-of-bounds crash after 816 seconds. The focused Responses tests and typecheck remain green, so this PR stays draft for CI confirmation.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed. No user-facing configuration or API contract changed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults. No credential or logging path changed.

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