feat: add grok-stt tool (speech-to-text)

[hrco]

Summary

• PR 2 of 5 for issue feat: new tool areas — TTS, STT, Files API, Context Compaction, Batch #21 — adds the grok-stt tool per docs/issue-21-scope.md (from docs: scope issue #21 (TTS, STT, Files API, Context Compaction, Batch) #34).
• New constant: STT_API_BASE.
• make_stt_request() — submits via the url field only (server-side download). The endpoint requires multipart/form-data even for url-only submissions; httpx's plain data= dict would encode as application/x-www-form-urlencoded instead, so fields go through files= using (None, value) tuples to force multipart encoding without an actual file part.
• handle_grok_stt() — validates audio_url, guards that inverse_text_normalization requires language (per the API's documented constraint), returns transcript text plus a speaker/channel table when diarize or multichannel was requested.
• Tool schema registered in list_tools() / dispatched in call_tool().

v1 cuts (flagged in the scope doc as intentional, not oversights):

• Local file upload deferred — url-only for now.
• keyterm biasing deferred — delimiter format is unconfirmed in xAI's docs.

Note on test file naming: used tests/test_stt_tools.py instead of the scope doc's suggested combined test_audio_tools.py — PR 1 (grok-tts, #35) already claims that filename, and since these PRs merge independently, sharing a filename would cause an add/add conflict for whichever merges second.

Test plan

• pytest tests/test_stt_tools.py -v — 9/9 new tests pass
• pytest tests/ — 212 passed, 54 pre-existing failures (51 known pytest-asyncio strict-mode baseline + 3 stale Docker-namespace assertions from the fix: publish Docker image under nexuswedge namespace #27 nexuswedge rename, both unrelated to this change), 3 skipped — no regressions
• Verified grok-stt appears in list_tools() output (20 tools total)
• Founder smoke test against real XAI_API_KEY + real audio URL

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📥 Commits

Reviewing files that changed from the base of the PR and between e7ee108 and e0e2e64.

📒 Files selected for processing (2)

• tests/test_stt_tools.py
• xbridge_mcp/server.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/grok-stt

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