fix(translator): strip neutral tool_choice when tools absent in Responses-to-Chat (#12141)

[HouMinXi]

Summary

Closes #12141.

When clients invoke the /v1/responses endpoint with neutral tool_choice ("auto" or "none") without providing tools (or passing tools: []), translating to Chat Completions (openaiResponsesToOpenAIRequest) previously preserved tool_choice verbatim. Strict Chat Completions backends (such as vLLM, Ollama, and strict OpenAI-compatible engines) reject requests containing tool_choice without tools with HTTP 400: When using tool_choice, tools must be set.

Changes

• open-sse/translator/request/openai-responses.ts: In openaiResponsesToOpenAIRequest, after tool extraction and filtering, when finalChatTools.length === 0 and result.tool_choice is "auto" or "none", delete result.tool_choice. Contradictory choices ("required" or explicit function objects) remain preserved so upstream contracts remain transparent.
• tests/unit/responses-to-chat-no-tools-tool-choice-12141.test.ts: Dedicated test suite verifying empty tools with auto, none, valid tools preservation, and contradictory choice retention.
• tests/unit/responses-translation-fixes.test.ts: Updated existing test case to verify both with-tools preservation and without-tools stripping.

Verification

• npm run typecheck:core: 0 errors.
• npm run check:cycles: PASS (no cycles).
• node --test tests/unit/responses-to-chat-no-tools-tool-choice-12141.test.ts tests/unit/responses-translation-fixes.test.ts: 48/48 PASS.
• Regression suite: all translator tests (76/76) PASS.

[diegosouzapw]

Validated in local merge-train on 192.168.0.113 — train of #12258 #12262 #12166 #12281 #11259 #11950 merged clean onto origin/release/v3.8.51 (f5e7095):

• Run 1 (/opt/actions-runner-omniroute-5, train tip 85cd9119): typecheck:core, file-size, complexity ×2, changelog-integrity — all green; the test:unit step was killed by a CI job landing on that runner mid-train (workspace clobbered — infra, documented risk).
• Run 2 (/srv/omniroute-train/.claude/worktrees/mt-green1, same 6 PRs re-boarded, fresh npm ci): unit 35768/35805 pass (/tmp/mt-unit2.log), vitest 464/465 (/tmp/mt-vitest2.log).
• Every failure is a latency/timing assert (bounded-time, event-loop lag, cooldown windows) on a box that was never idle (3 active CI runner workers throughout). Discriminated per merge-gates §3: the 3 persistent titles reproduce identically on the pure base tip on the same box (/tmp/mt-base-isolated.log, BASE_ISOLATED_EXIT=1 — same tests, same asserts), 5 more titles reproduce on the pure base on the devbox, and the single vitest failure (provider-family-combos) also fails on main's nightly without any of these PRs. Inherited/infra — not introduced by this train.