fix(responses): encode large native Codex HTTP uploads as bytes

[MaxyMilan]

Summary

Codex conversations containing many images can repeatedly fail with 429 upstream_reset_replay_refused after falling back from WebSocket to HTTP. In a macOS reproduction with a roughly 25 MB synthetic image request, Bun's string-body upload failed with ECONNRESET before response headers. Sending the serialized JSON as a UTF-8 buffer succeeded; the patched OpenCodex proxy then completed the same size request with response.completed in about five seconds.

Encode native ChatGPT Responses and compact HTTP string bodies of at least 1 MiB at the final physical send. This leaves WebSocket selection, request contents, headers, cancellation, destination selection and replay refusal unchanged. Other endpoints and existing byte/stream bodies keep their current representation.

Add regression coverage for Unicode byte preservation, rebuilt compact destinations, unchanged small/non-native requests, and nested dispatch without extra sends. Update the transport and user documentation.

Verification

• Before the fix, three new regression cases failed; after it, all 32 focused transport tests passed: bun test tests/responses/responses-fetch-helpers-boundary.test.ts tests/responses/fresh-connection-optout.test.ts tests/responses/provider-egress-fetch.test.ts.
• bun run typecheck — passed.
• bun run privacy:scan — passed.
• bun run structure:check — passed.
• ./node_modules/bun/bin/bun.exe scripts/test.ts tests/server/server-management-auth.test.ts — 48 passed, 0 failed with the fix applied.
• cd docs-site && bun install --frozen-lockfile && bun run build — passed; 561 pages and 77,923 internal links checked.
• Live reproduction on Bun 1.4.0: large string upload failed; byte-buffer upload and the patched local proxy completed successfully. No private conversation contents or credentials are included here.
• ./node_modules/bun/bin/bun.exe run test:changed on Bun 1.4.0 selected 484 files: 10,905 passed, 34 skipped, 32 failed. The three failures in native-codex-toggle.test.ts, injection-model-suggest-routes.test.ts, and package-tree-integrity.test.ts also reproduce with the unmodified upstream transport. The other 29 were a timeout in server-management-auth.test.ts followed by spend-ledger ownership conflicts; that file passes in isolation with this patch.
• For comparison, the same 484-file selection on unmodified upstream dev (3bae88cce7400e47a5e69f9fd28749ed25f919f6) produced 10,906 passed, 34 skipped, 26 failed, including different failures. Neither broad run is green; the failure sets differ and are not being represented as fully classified or equivalent.

The full repository suite was not run. This PR remains a draft for maintainer review and CI.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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• Required local validation passed; commands, results, and any full-suite exception are documented.

• I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).

• I resolved all correct Codex and CodeRabbit findings.

• My PR is ready for review.

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✅ Deterministic PR hygiene checks passed.

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• Tick all four boxes in the PR description once you're done (currently 0/4).

Review readiness checklist

• ⬜ Required local validation passed; commands, results, and any full-suite exception are documented.
• ⬜ I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).
• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

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Hygiene

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[lidge-jun]

Superseded by #6513, now merged into dev as e77bfb4901d405724edc4295caf5d9f0675a672e. The native large-string-to-UTF-8-byte conversion, source regression cases and documentation were carried, with additional threshold, rewritten-destination and abort tests plus precise threshold wording. No implementation part was dropped. Original author/commit provenance and Co-authored-by credit are retained.

The landed head passed required scoped CI: https://github.com/lidge-jun/opencodex/actions/runs/37130246238. Full integrated cross-platform validation and production publication are still upcoming; this closure records delivery to dev, not a release or a new live native-upload acceptance claim. Thank you for the original reproduction and fix.