fix(transcription): harden language parameter validation and tests

- Add ISO-639 pattern validation (2-3 lowercase letters) to schema
- Normalize empty language to None in provider constructors
- Extract shared httpx mock stubs, parameterize provider tests
- Add test for language=None omitting field from multipart body
- Add test for Pydantic pattern validation rejecting invalid codes

nanobot/config/schema.py

@@ -29,7 +29,7 @@ class ChannelsConfig(Base):
     send_tool_hints: bool = False  # stream tool-call hints (e.g. read_file("…"))
     send_max_retries: int = Field(default=3, ge=0, le=10)  # Max delivery attempts (initial send included)
     transcription_provider: str = "groq"  # Voice transcription backend: "groq" or "openai"
-    transcription_language: str | None = None  # Optional ISO-639-1 hint for audio transcription
+    transcription_language: str | None = Field(default=None, pattern=r"^[a-z]{2,3}$")  # Optional ISO-639-1 hint for audio transcription
 
 
 class DreamConfig(Base):

nanobot/providers/transcription.py

@@ -22,7 +22,7 @@ class OpenAITranscriptionProvider:
             or os.environ.get("OPENAI_TRANSCRIPTION_BASE_URL")
             or "https://api.openai.com/v1/audio/transcriptions"
         )
-        self.language = language
+        self.language = language or None
 
     async def transcribe(self, file_path: str | Path) -> str:
         if not self.api_key:
@@ -64,7 +64,7 @@ class GroqTranscriptionProvider:
     ):
         self.api_key = api_key or os.environ.get("GROQ_API_KEY")
         self.api_url = api_base or os.environ.get("GROQ_BASE_URL") or "https://api.groq.com/openai/v1/audio/transcriptions"
-        self.language = language
+        self.language = language or None
 
     async def transcribe(self, file_path: str | Path) -> str:
         """