fix(voice): treat a non-list record_keyterms argument as no change

[Lesereingrape]

What

_parse_tool_call assumed each of pending / confirm / remove decodes to a list. A value
that is not a list is now ignored, the way blank and non-string items already were.

Why

The arguments come straight from a provider, through parse_function_arguments, which
deliberately recovers malformed JSON rather than rejecting it — and data.get(key, []) then
iterates whatever the model actually sent:

• {"pending": null} is valid JSON, so it reaches _terms as None and raises
  TypeError: 'NoneType' object is not iterable. KeytermDetector._run_once is decorated with
  @utils.log_exceptions, which logs and re-raises, so the pass dies and that turn's keyterm
  update is lost.
• A truncated call such as {"confirm": "Kubernetes is repaired by json_repair into the string
  "Kubernetes", and iterating a string yields its characters: _apply_pass confirms K, u,
  b, …, and _run_once pushes that set to _stt._update_session_keyterms()
  (keyterm_detection.py:386-387). The detector's own instructions treat a wrong spelling as
  unrecoverable for the rest of the call, so ignoring a group the recognizer cannot use fits
  "precision beats coverage" better than biasing on single letters.

Both reproduce on main at 98ed3e8:

null  -> raised TypeError 'NoneType' object is not iterable
trunc -> ([], ['K', 'u', 'b', 'e', 'r', 'n', 'e', 't', 'e', 's'], [])

Tests

tests/test_stt_context.py, which already covers _parse_tool_call:

• before the fix: 1 failed, 36 passed in 5.47s (the TypeError above)
• after: 37 passed in 0.60s
• keyterm-related unit modules (test_keyterm_detection_span, test_stt_context,
  test_agent_session, test_meta_stt, test_session_options_report, test_trace_types_pii,
  --unit -n auto): 1 failed, 155 passed, 1 error. The failure is test_trace_types_pii reading
  a source file with the Windows default codec, and the error is test_meta_stt importing a plugin
  that is not installed here; both reproduce on the untouched base and neither touches a file in
  this diff.
• ruff check / ruff format --check: clean on both files. mypy in strict mode reports no error
  in keyterm_detection.py.

Environment note: Windows, core package installed editable. uv sync --all-extras cannot resolve
bithuman here (no Windows wheel), so plugin packages are absent and I could not run the full
--unit --audio_eot gate locally.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[longcw]

lgtm!