fix(deps): unbreak [all] install — drop mistralai while PyPI quarantined (#24205)

The `mistralai` PyPI package was quarantined on 2026-05-12 after a
malicious 2.4.6 release. Every fresh resolve (AUR makepkg, Docker build,
CI run, install.sh first-run) currently fails on
`mistralai>=2.3.0,<3` because PyPI returns zero candidates.

Existing users running `hermes update` mostly didn't notice — `hermes
update` falls back from `.[all]` to per-extra retries and silently
skips mistral with a warning that scrolls past. But fresh installs
hard-fail or lose every other extra.

Changes:
- pyproject.toml: drop `hermes-agent[mistral]` from `[all]` and
  `[termux-all]`. The `mistral` extra itself is preserved so users
  can opt back in once PyPI un-quarantines.
- hermes_cli/tools_config.py: hide Mistral Voxtral TTS from the
  `hermes tools` provider picker until restored.
- hermes_cli/web_server.py: drop "mistral" from dashboard STT options.
- tools/transcription_tools.py: explicit `provider: mistral` returns
  "none" with a clear status message; auto-detect skips mistral.
- tools/tts_tool.py: dispatcher returns a clear "temporarily disabled"
  error before any SDK import attempt (avoids cached-stale-package
  surprises).
- tests/tools/: update three test files to assert the new disabled
  behavior. Each test docstring records why and points at the rollback
  trigger (PyPI un-quarantines mistralai).

Restore plan: revert this commit once the package is available on PyPI
again. The behavior change is intentional and documented in code
comments + test docstrings to make the rollback trivial.

Validation:
- scripts/run_tests.sh tests/tools/ -k 'mistral or stt or tts' →
  425/425 passing.

Refs: https://pypi.org/simple/mistralai/ (currently
"pypi:project-status: quarantined").

hermes_cli/web_server.py

@@ -273,7 +273,9 @@ _SCHEMA_OVERRIDES: Dict[str, Dict[str, Any]] = {
     "stt.provider": {
         "type": "select",
         "description": "Speech-to-text provider",
-        "options": ["local", "openai", "mistral"],
+        # "mistral" temporarily removed — mistralai PyPI package quarantined
+        # (malicious 2.4.6 release on 2026-05-12). Restore once available.
+        "options": ["local", "openai"],
     },
     "display.skin": {
         "type": "select",