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:
         """

tests/channels/test_channel_plugins.py

@@ -334,21 +334,24 @@ async def test_base_channel_passes_language_to_groq_transcription_provider():
     assert captured["language"] == "ko"
 
 
-@pytest.mark.asyncio
+# ---------------------------------------------------------------------------
-async def test_groq_transcription_provider_includes_language(tmp_path):
+# Transcription provider HTTP tests
-    from nanobot.providers.transcription import GroqTranscriptionProvider
+# ---------------------------------------------------------------------------
 
-    audio = tmp_path / "sample.wav"
+from nanobot.providers.transcription import GroqTranscriptionProvider as _GroqProvider
-    audio.write_bytes(b"audio")
+from nanobot.providers.transcription import OpenAITranscriptionProvider as _OpenAIProvider
-    captured: dict[str, object] = {}
 
-    class _Response:
-        def raise_for_status(self):
-            return None
 
-        def json(self):
+class _StubResponse:
-            return {"text": "hello"}
+    def raise_for_status(self):
+        return None
 
+    def json(self):
+        return {"text": "hello"}
+
+
+def _stub_async_client(captured: dict[str, object]):
+    """Return an httpx.AsyncClient stub that records POST calls into *captured*."""
     class _AsyncClient:
         async def __aenter__(self):
             return self
@@ -357,19 +360,50 @@ async def test_groq_transcription_provider_includes_language(tmp_path):
             return False
 
         async def post(self, url, headers=None, files=None, timeout=None):
-            captured["url"] = url
-            captured["headers"] = headers
             captured["files"] = files
-            captured["timeout"] = timeout
+            return _StubResponse()
-            return _Response()
 
-    provider = GroqTranscriptionProvider(api_key="k", language="ko")
+    return _AsyncClient()
 
-    with patch("nanobot.providers.transcription.httpx.AsyncClient", return_value=_AsyncClient()):
+
+@pytest.mark.parametrize(
+    "provider_cls,language",
+    [(_GroqProvider, "ko"), (_OpenAIProvider, "en")],
+    ids=["groq", "openai"],
+)
+@pytest.mark.asyncio
+async def test_transcription_provider_includes_language(tmp_path, provider_cls, language):
+    """Provider must include the 'language' field in multipart body when set."""
+    audio = tmp_path / "sample.wav"
+    audio.write_bytes(b"audio")
+    captured: dict[str, object] = {}
+
+    with patch("nanobot.providers.transcription.httpx.AsyncClient", return_value=_stub_async_client(captured)):
+        provider = provider_cls(api_key="k", language=language)
         result = await provider.transcribe(audio)
 
     assert result == "hello"
-    assert captured["files"]["language"] == (None, "ko")
+    assert captured["files"]["language"] == (None, language)
+
+
+@pytest.mark.parametrize(
+    "provider_cls",
+    [_GroqProvider, _OpenAIProvider],
+    ids=["groq", "openai"],
+)
+@pytest.mark.asyncio
+async def test_transcription_provider_omits_language_when_none(tmp_path, provider_cls):
+    """When language is not set, the 'language' key must be absent from the multipart body."""
+    audio = tmp_path / "sample.wav"
+    audio.write_bytes(b"audio")
+    captured: dict[str, object] = {}
+
+    with patch("nanobot.providers.transcription.httpx.AsyncClient", return_value=_stub_async_client(captured)):
+        provider = provider_cls(api_key="k")
+        result = await provider.transcribe(audio)
+
+    assert result == "hello"
+    assert "language" not in captured["files"]
 
 
 def test_channels_login_uses_discovered_plugin_class(monkeypatch):
@@ -530,6 +564,24 @@ def test_channels_config_send_max_retries_upper_bound():
         ChannelsConfig(send_max_retries=11)
 
 
+def test_channels_config_transcription_language_pattern():
+    """transcription_language must match ISO-639 format (2-3 lowercase letters) or be None."""
+    from pydantic import ValidationError
+
+    # Valid values
+    assert ChannelsConfig(transcription_language="en").transcription_language == "en"
+    assert ChannelsConfig(transcription_language="kor").transcription_language == "kor"
+    assert ChannelsConfig(transcription_language=None).transcription_language is None
+
+    # Invalid values
+    with pytest.raises(ValidationError):
+        ChannelsConfig(transcription_language="EN")       # uppercase
+    with pytest.raises(ValidationError):
+        ChannelsConfig(transcription_language="english")   # full word
+    with pytest.raises(ValidationError):
+        ChannelsConfig(transcription_language="en-US")     # BCP 47 tag
+
+
 # ---------------------------------------------------------------------------
 # _send_with_retry
 # ---------------------------------------------------------------------------