fix(speak): honour the voice profile's language instead of forcing English

Both speak surfaces built their GenerationRequest with a hardcoded "en"
fallback and never consulted the resolved profile, so a profile created
with language="fr" was still synthesised as English unless the caller
passed language= explicitly.

This hurts the MCP path most: an agent calling voicebox.speak has no way
to know the bound profile's language, so it cannot pass the argument
either. Every agent-triggered generation on a non-English profile came
out with an English accent.

The fallback chain is now explicit argument -> resolved profile's
language -> "en", which matches how engine and personality already
consult the resolved binding. The "en" backstop is kept so profiles with
no language set behave exactly as before.

Adds backend/tests/test_speak_language.py covering both surfaces: the
fallback, explicit-argument precedence, and the unchanged "en" default.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

backend/tests/test_speak_language.py

@@ -0,0 +1,163 @@
+"""Tests for voice-profile language fallback on the two speak surfaces.
+
+Both speak paths built their ``GenerationRequest`` with a hardcoded ``"en"``
+fallback and never consulted the resolved profile, so a profile created with
+``language="fr"`` was still synthesised as English unless every caller passed
+``language=`` explicitly. Agents going through MCP had no way to know the
+profile's language, so they couldn't pass it either.
+
+These tests pin the fix: the fallback chain is now explicit argument →
+resolved profile's language → ``"en"``, matching how ``engine`` and
+``personality`` already consult the resolved binding.
+"""
+
+import pytest
+
+import backend.routes.generations as generations
+import backend.routes.speak as speak_route
+from backend import models
+from backend.mcp_server import tools
+
+
+class _FakeGeneration:
+    """Minimal stand-in for GenerationResponse consumed by the speak paths."""
+
+    id = "gen-test"
+    status = "generating"
+
+    def model_dump(self, mode="json"):
+        return {"id": self.id, "status": self.status}
+
+
+class _FakeProfile:
+    def __init__(self, language):
+        self.id = "p1"
+        self.name = "Siwis"
+        self.language = language
+        self.personality = None
+
+
+class _FakeQuery:
+    def filter(self, *args, **kwargs):
+        return self
+
+    def first(self):
+        # No per-client binding — engine/personality fall through to their
+        # own defaults, leaving language as the only variable under test.
+        return None
+
+
+class _FakeDB:
+    def query(self, *args, **kwargs):
+        return _FakeQuery()
+
+    def close(self):
+        pass
+
+
+class _FakeRequest:
+    """Stands in for starlette's Request — only headers are read."""
+
+    def __init__(self, client_id=None):
+        self.headers = {"X-Voicebox-Client-Id": client_id} if client_id else {}
+
+
+@pytest.fixture
+def captured_request(monkeypatch):
+    """Capture the GenerationRequest instead of running a real generation.
+
+    Both speak paths import ``generate_speech`` lazily from
+    ``routes.generations``, so patching the attribute on that module
+    intercepts the call on either surface.
+    """
+    captured = {}
+
+    async def fake_generate_speech(req, db):
+        captured["req"] = req
+        return _FakeGeneration()
+
+    monkeypatch.setattr(generations, "generate_speech", fake_generate_speech)
+    monkeypatch.setattr(speak_route.mcp_events, "publish", lambda *a, **k: None)
+    monkeypatch.setattr(tools.mcp_events, "publish", lambda *a, **k: None)
+    return captured
+
+
+# ─── REST: POST /speak ────────────────────────────────────────────────────
+
+
+async def _call_rest(monkeypatch, profile_language, requested_language=None):
+    monkeypatch.setattr(
+        speak_route,
+        "resolve_profile",
+        lambda profile, client_id, db: _FakeProfile(profile_language),
+    )
+    await speak_route.speak(
+        models.SpeakRequest(text="Bonjour", language=requested_language),
+        _FakeRequest(client_id="claude-code"),
+        _FakeDB(),
+    )
+
+
+async def test_rest_speak_falls_back_to_profile_language(
+    captured_request, monkeypatch
+):
+    await _call_rest(monkeypatch, profile_language="fr")
+    assert captured_request["req"].language == "fr"
+
+
+async def test_rest_speak_explicit_language_wins(captured_request, monkeypatch):
+    # An explicit argument still overrides the profile — a French profile can
+    # be asked to read an English string.
+    await _call_rest(monkeypatch, profile_language="fr", requested_language="en")
+    assert captured_request["req"].language == "en"
+
+
+async def test_rest_speak_defaults_to_en_without_profile_language(
+    captured_request, monkeypatch
+):
+    # Profiles predating the language column resolve to None; the "en"
+    # backstop keeps their behaviour unchanged.
+    await _call_rest(monkeypatch, profile_language=None)
+    assert captured_request["req"].language == "en"
+
+
+# ─── MCP: voicebox.speak ──────────────────────────────────────────────────
+
+
+async def _call_mcp(monkeypatch, profile_language, requested_language=None):
+    from mcp.server.fastmcp import FastMCP
+
+    monkeypatch.setattr(
+        tools,
+        "resolve_profile",
+        lambda profile, client_id, db: _FakeProfile(profile_language),
+    )
+    monkeypatch.setattr(tools, "get_db", lambda: iter([_FakeDB()]))
+
+    mcp = FastMCP("test")
+    tools.register_tools(mcp)
+    args = {"text": "Bonjour"}
+    if requested_language is not None:
+        args["language"] = requested_language
+    await mcp.call_tool("voicebox.speak", args)
+
+
+async def test_mcp_speak_falls_back_to_profile_language(
+    captured_request, monkeypatch
+):
+    # The agent-facing path matters most: an MCP client can't know the
+    # profile's language, so omitting it must not silently mean English.
+    await _call_mcp(monkeypatch, profile_language="fr")
+    assert captured_request["req"].language == "fr"
+
+
+async def test_mcp_speak_explicit_language_wins(captured_request, monkeypatch):
+    await _call_mcp(monkeypatch, profile_language="fr", requested_language="en")
+    assert captured_request["req"].language == "en"
+
+
+async def test_mcp_speak_defaults_to_en_without_profile_language(
+    captured_request, monkeypatch
+):
+    await _call_mcp(monkeypatch, profile_language=None)
+    assert captured_request["req"].language == "en"