fix(tui): address Copilot round-3 review on #19835

Three classes of robustness issue caught on the second pass — all
revolve around malformed YAML tipping ``parseVoiceRecordKey`` or
``_voice_record_key`` into a crash instead of the documented
fallback.

* **Parser crashed on non-string YAML scalars.** ``config.get full``
  returns raw ``yaml.safe_load`` output, so ``voice.record_key: 1``
  or ``voice.record_key: true`` in a hand-edited config would hit
  ``.trim()`` on a number/bool and throw, breaking startup and
  every mtime re-apply. Accept ``unknown`` at the signature, guard
  with ``typeof raw !== 'string'``, and fall back to the default.

* **Backend blew up on non-dict ``voice:``.** Same YAML hazard on
  the gateway side: ``voice: true`` / ``voice: cmd+b`` left
  ``_load_cfg().get("voice")`` as a bool/str, so ``.get("record_key")``
  raised AttributeError and took every ``voice.toggle`` branch down
  with it. Centralised the lookup in a single
  ``_voice_record_key()`` helper that ``isinstance``-guards both
  ``voice`` and ``record_key`` and falls back to ``ctrl+b``.

* **Multi-modifier chords silently dropped extras.** The previous
  validator only checked the first modifier token, so ``ctrl+alt+r``
  silently parsed as ``ctrl+r`` and ``cmd+ctrl+b`` as ``super+b`` —
  a typo bound a different shortcut than the user configured.
  Reject multi-modifier spellings outright; the classic CLI only
  supports single-modifier bindings via prompt_toolkit's ``c-x`` /
  ``a-x`` rewrite, so this matches CLI parity.

Coverage added:

* ``parseVoiceRecordKey`` fallback on ``1`` / ``true`` / ``null`` /
  ``undefined`` / ``{}``.
* ``parseVoiceRecordKey`` fallback on ``ctrl+alt+r`` /
  ``cmd+ctrl+b`` / ``alt+ctrl+space``.
* ``test_voice_toggle_handles_non_dict_voice_cfg`` exercises
  every non-dict ``voice:`` shape (bool, str, None, int, list) and
  asserts each falls back to ``record_key: 'ctrl+b'``.

Suite: 581/581 TUI vitest green, 3/3 backend voice tests green,
tsc --noEmit clean.

tui_gateway/server.py

@@ -5278,6 +5278,22 @@ def _voice_tts_enabled() -> bool:
     return os.environ.get("HERMES_VOICE_TTS", "").strip() == "1"
 
 
+def _voice_record_key() -> str:
+    """Current ``voice.record_key`` value, documented default on error.
+
+    ``_load_cfg()`` returns raw ``yaml.safe_load()`` output, so ``voice``
+    may be any scalar — a hand-edited ``voice: true`` or ``voice: cmd+b``
+    (string where a dict is expected) would break ``.get("record_key")``
+    and take every ``voice.toggle`` branch down with it (Copilot round-3
+    review on #19835). Coerce through ``isinstance`` so malformed config
+    falls back to the documented default instead of crashing /voice.
+    """
+    voice_cfg = _load_cfg().get("voice")
+    record_key = voice_cfg.get("record_key") if isinstance(voice_cfg, dict) else None
+
+    return str(record_key) if isinstance(record_key, str) and record_key else "ctrl+b"
+
+
 @method("voice.toggle")
 def _(rid, params: dict) -> dict:
     """CLI parity for the ``/voice`` slash command.
@@ -5304,9 +5320,7 @@ def _(rid, params: dict) -> dict:
         # ignored the config (#18994).
         payload: dict = {
             "enabled": _voice_mode_enabled(),
-            "record_key": str(
-                (_load_cfg().get("voice") or {}).get("record_key") or "ctrl+b"
-            ),
+            "record_key": _voice_record_key(),
             "tts": _voice_tts_enabled(),
         }
         try:
@@ -5347,9 +5361,7 @@ def _(rid, params: dict) -> dict:
             rid,
             {
                 "enabled": enabled,
-                "record_key": str(
-                    (_load_cfg().get("voice") or {}).get("record_key") or "ctrl+b"
-                ),
+                "record_key": _voice_record_key(),
                 "tts": _voice_tts_enabled(),
             },
         )
@@ -5368,9 +5380,7 @@ def _(rid, params: dict) -> dict:
             rid,
             {
                 "enabled": True,
-                "record_key": str(
-                    (_load_cfg().get("voice") or {}).get("record_key") or "ctrl+b"
-                ),
+                "record_key": _voice_record_key(),
                 "tts": new_value,
             },
         )