Broad service settings refactor, with the primary aim of making service settings discoverable and strongly-typed. Service settings can be updated at runtime with *UpdateSettingsFrames.

Does not (yet) touch `InputParams`, to avoid scope creep and touching something currently part of the public API. But there is a lot of overlap between `*Settings` object fields and `InputParams` fields.

Other than discoverability/typing, these are some other improvements brought by this refactor:
- There is now a single code path (see `_update_settings_from_typed`) where services can respond to settings changes (by, say, reconnecting if needed), improving maintainability and guaranteeing one and only one reconnection no matter which settings changed
- `set_language`/`set_model`/`set_voice`—which we're assuming are usable as public methods, though *not* recommended over `*UpdateSettingsFrame`—all use the same code path as settings updates. They're also now all consistent in that, if a service needs to respond to a change (by, say, reconnecting if needed), any of these methods will kick off that process. Note that this is technically a behavior change.
- Several services now properly react to changed settings by reconnecting:
  - `AWSTranscribeSTTService`
  - `AzureSTTService`
  - `SonioxSTTService`
  - `GladiaSTTService`
  - `SpeechmaticsSTTService`
  - `AssemblyAISTTService`
  - `CartesiaSTTService`
  - `FishAudioTTSService` (would previously only reconnect when `model` changed)
  - `GoogleSTTService`
  - `SpeechmaticsSTTService` (which previously only handled *some* settings updates through a nonstandard public `update_params` method)
  - `GradiumSTTService`
  - `NvidiaSegmentedSTTService` (which previously only handled changes to language)
- Bookkeeping across various services has been reduced, mostly by deduping ivars; the `self._settings` ivar is treated as the source of truth

NOTE: I pretty much guarantee that there are services missed in this PR in terms of bringing to consistency with how updates are handled (like whether changes in certain fields trigger reconnects when they need to). We can squash remaining inconsistencies as we stumble onto them, service by service. The goal here is to get things *mostly* in order, and establish the infrastructure and patterns we'll need going forward.

src/pipecat/services/openai/stt.py

@@ -16,6 +16,7 @@ Provides two STT services:
 
 import base64
 import json
+from dataclasses import dataclass, field
 from typing import AsyncGenerator, Literal, Optional, Union
 
 from loguru import logger
@@ -34,6 +35,7 @@ from pipecat.frames.frames import (
     VADUserStoppedSpeakingFrame,
 )
 from pipecat.processors.frame_processor import FrameDirection
+from pipecat.services.settings import NOT_GIVEN, STTSettings, is_given
 from pipecat.services.stt_latency import OPENAI_REALTIME_TTFS_P99, OPENAI_TTFS_P99
 from pipecat.services.stt_service import WebsocketSTTService
 from pipecat.services.whisper.base_stt import BaseWhisperSTTService, Transcription
@@ -123,6 +125,17 @@ class OpenAISTTService(BaseWhisperSTTService):
 _OPENAI_SAMPLE_RATE = 24000
 
 
+@dataclass
+class OpenAIRealtimeSTTSettings(STTSettings):
+    """Typed settings for the OpenAI Realtime STT service.
+
+    Parameters:
+        prompt: Optional prompt text to guide transcription style.
+    """
+
+    prompt: Optional[str] = field(default_factory=lambda: NOT_GIVEN)
+
+
 class OpenAIRealtimeSTTService(WebsocketSTTService):
     """OpenAI Realtime Speech-to-Text service using WebSocket transcription sessions.
 
@@ -213,12 +226,17 @@ class OpenAIRealtimeSTTService(WebsocketSTTService):
         self._base_url = base_url
         self.set_model_name(model)
 
-        self._language_code = self._language_to_code(language) if language else None
         self._prompt = prompt
         self._turn_detection = turn_detection
         self._noise_reduction = noise_reduction
         self._should_interrupt = should_interrupt
 
+        self._settings: OpenAIRealtimeSTTSettings = OpenAIRealtimeSTTSettings(
+            model=model,
+            language=language,
+            prompt=prompt,
+        )
+
         self._receive_task = None
         self._session_ready = False
         self._resampler = create_stream_resampler()
@@ -248,19 +266,31 @@ class OpenAIRealtimeSTTService(WebsocketSTTService):
         """
         return True
 
-    async def set_language(self, language: Language):
-        """Set the language for speech recognition.
+    async def _update_settings_from_typed(self, update: STTSettings) -> set[str]:
+        """Apply a typed settings update and send session update if needed.
 
-        If the session is already active, sends an updated configuration
-        to the server.
+        Keeps ``_language_code`` and ``_prompt`` in sync with typed settings
+        and sends a ``session.update`` to the server when the session is active.
 
         Args:
-            language: The language to use for speech recognition.
+            update: A :class:`STTSettings` (or ``OpenAIRealtimeSTTSettings``) delta.
+
+        Returns:
+            Set of field names whose values actually changed.
         """
-        self._language_code = self._language_to_code(language)
+        changed = await super()._update_settings_from_typed(update)
+
+        if not changed:
+            return changed
+
+        if "prompt" in changed and isinstance(self._settings, OpenAIRealtimeSTTSettings):
+            self._prompt = self._settings.prompt
+
         if self._session_ready:
             await self._send_session_update()
 
+        return changed
+
     async def start(self, frame: StartFrame):
         """Start the service and establish WebSocket connection.
 
@@ -407,8 +437,11 @@ class OpenAIRealtimeSTTService(WebsocketSTTService):
         """Send ``session.update`` to configure the transcription session."""
         transcription: dict = {"model": self.model_name}
 
-        if self._language_code:
-            transcription["language"] = self._language_code
+        language_code = (
+            self._language_to_code(self._settings.language) if self._settings.language else None
+        )
+        if language_code:
+            transcription["language"] = language_code
 
         if self._prompt:
             transcription["prompt"] = self._prompt