fix(transcription): honor api_base for OpenAI transcription provider

Complete the symmetry left by #3214: ChannelManager._resolve_transcription_base
already resolves providers.openai.api_base, but BaseChannel.transcribe_audio
instantiated OpenAITranscriptionProvider without forwarding it, and the provider
__init__ did not accept the parameter. Self-hosted OpenAI-compatible Whisper
endpoints (LiteLLM, vLLM, etc.) configured via config.json were therefore
ignored for the OpenAI backend.

- OpenAITranscriptionProvider.__init__ now accepts api_base with env fallback
  (OPENAI_TRANSCRIPTION_BASE_URL) matching the Groq pattern.
- BaseChannel.transcribe_audio forwards self.transcription_api_base to OpenAI.
- Tests mirror the existing Groq coverage: manager propagation for provider
  "openai", BaseChannel-to-provider argument passing, and provider default vs
  override for api_url.

Fully backward-compatible: when api_base is None and the env var is unset,
the default https://api.openai.com/v1/audio/transcriptions is used.

Refs #3213, follow-up to #3214.

nanobot/channels/base.py

@@ -45,7 +45,10 @@ class BaseChannel(ABC):
         try:
             if self.transcription_provider == "openai":
                 from nanobot.providers.transcription import OpenAITranscriptionProvider
-                provider = OpenAITranscriptionProvider(api_key=self.transcription_api_key)
+                provider = OpenAITranscriptionProvider(
+                    api_key=self.transcription_api_key,
+                    api_base=self.transcription_api_base or None,
+                )
             else:
                 from nanobot.providers.transcription import GroqTranscriptionProvider
                 provider = GroqTranscriptionProvider(

nanobot/providers/transcription.py

@@ -10,9 +10,13 @@ from loguru import logger
 class OpenAITranscriptionProvider:
     """Voice transcription provider using OpenAI's Whisper API."""
 
-    def __init__(self, api_key: str | None = None):
+    def __init__(self, api_key: str | None = None, api_base: str | None = None):
         self.api_key = api_key or os.environ.get("OPENAI_API_KEY")
-        self.api_url = "https://api.openai.com/v1/audio/transcriptions"
+        self.api_url = (
+            api_base
+            or os.environ.get("OPENAI_TRANSCRIPTION_BASE_URL")
+            or "https://api.openai.com/v1/audio/transcriptions"
+        )
 
     async def transcribe(self, file_path: str | Path) -> str:
         if not self.api_key: