fix(tts): restore 3.28 schema compatibility and fallback observability (#57953)

* fix(tts): restore legacy config compatibility and fallback observability

* fix(tts): surface fallback attempts in status and telephony

* test(tts): cover /tts audio to /tts status fallback flow

* docs(tts): align migration and fallback observability guidance

* TTS: redact fallback logs and scope legacy plugin migration

* Infra: dedupe UV_EXTRA_INDEX_URL in host env policy

* Docs: scope doctor TTS migration to voice-call

* voice-call: restore strict known TTS provider validation

docs/tts.md

@@ -219,6 +219,7 @@ Then run:
 - `modelOverrides`: allow the model to emit TTS directives (on by default).
   - `allowProvider` defaults to `false` (provider switching is opt-in).
 - `providers.<id>`: provider-owned settings keyed by speech provider id.
+- Legacy direct provider blocks (`messages.tts.openai`, `messages.tts.elevenlabs`, `messages.tts.microsoft`, `messages.tts.edge`) are auto-migrated to `messages.tts.providers.<id>` on load.
 - `maxTextLength`: hard cap for TTS input (chars). `/tts audio` fails if exceeded.
 - `timeoutMs`: request timeout (ms).
 - `prefsPath`: override the local prefs JSON path (provider/limit/summary).
@@ -391,6 +392,9 @@ Notes:
 - `off|always|inbound|tagged` are per‑session toggles (`/tts on` is an alias for `/tts always`).
 - `limit` and `summary` are stored in local prefs, not the main config.
 - `/tts audio` generates a one-off audio reply (does not toggle TTS on).
+- `/tts status` includes fallback visibility for the latest attempt:
+  - success fallback: `Fallback: <primary> -> <used>` plus `Attempts: ...`
+  - failure: `Error: ...` plus `Attempts: ...`
 
 ## Agent tool