i18n: add translate() helper backed by call_llm

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_i18n.py

@@ -42,3 +42,78 @@ def test_respond_in_clause_unknown_lang_falls_back_to_english():
     prompt assembly. Unknown codes map to no-suffix (English default)."""
     from app.services.i18n import respond_in_clause
     assert respond_in_clause("xx") == ""
+
+
+@pytest.mark.asyncio
+async def test_translate_happy_path(monkeypatch):
+    from unittest.mock import AsyncMock, MagicMock
+
+    from app.services import translation as mod
+    from app.services.openrouter import LogResult
+
+    monkeypatch.setattr(mod, "call_llm", AsyncMock(return_value=LogResult(
+        content="# Apertura\n\nIl mercato è in calo dello 0,4%.",
+        model="deepseek/deepseek-v4-flash",
+        prompt_tokens=300, completion_tokens=80, cost_usd=0.00002,
+    )))
+
+    client = MagicMock()
+    translated, llm_log = await mod.translate(
+        client, "# Open\n\nThe market is down 0.4%.", "it",
+    )
+    assert "Apertura" in translated
+    assert llm_log.model == "deepseek/deepseek-v4-flash"
+    assert llm_log.cost_usd == pytest.approx(0.00002)
+
+
+@pytest.mark.asyncio
+async def test_translate_strips_code_fences(monkeypatch):
+    """If the LLM wraps the output in ```markdown ... ```, strip it."""
+    from unittest.mock import AsyncMock, MagicMock
+
+    from app.services import translation as mod
+    from app.services.openrouter import LogResult
+
+    fenced = "```markdown\n# Titolo\n\nCorpo.\n```"
+    monkeypatch.setattr(mod, "call_llm", AsyncMock(return_value=LogResult(
+        content=fenced, model="m", prompt_tokens=10, completion_tokens=20, cost_usd=0.0,
+    )))
+
+    client = MagicMock()
+    translated, _ = await mod.translate(client, "# Title\n\nBody.", "it")
+    assert "```" not in translated
+    assert translated.startswith("# Titolo")
+
+
+@pytest.mark.asyncio
+async def test_translate_provider_failure_propagates(monkeypatch):
+    from unittest.mock import AsyncMock, MagicMock
+
+    from app.services import translation as mod
+
+    monkeypatch.setattr(mod, "call_llm", AsyncMock(side_effect=RuntimeError("upstream down")))
+
+    client = MagicMock()
+    with pytest.raises(RuntimeError, match="upstream down"):
+        await mod.translate(client, "# Title\n\nBody.", "it")
+
+
+@pytest.mark.asyncio
+async def test_translate_unknown_lang_returns_source_unchanged(monkeypatch):
+    """Defensive: an unknown lang code (or 'en') short-circuits without
+    calling the LLM. Callers shouldn't have to gate the call themselves."""
+    from unittest.mock import AsyncMock, MagicMock
+
+    from app.services import translation as mod
+    from app.services.openrouter import LogResult
+
+    call_mock = AsyncMock(return_value=LogResult(
+        content="should not be returned",
+        model="m", prompt_tokens=0, completion_tokens=0, cost_usd=0.0,
+    ))
+    monkeypatch.setattr(mod, "call_llm", call_mock)
+
+    client = MagicMock()
+    out, _ = await mod.translate(client, "Hello world.", "en")
+    assert out == "Hello world."
+    call_mock.assert_not_awaited()