feat(bot): GigaChat function-calling provider for the chat agent; never 500 on provider errors

OpenRouter returns 403 «Access denied by security policy» from the RF server, and parsing that string error
crashed the request (500 instead of the fallback answer). Error parsing is robust now and any agent exception
degrades to the extractive answer. GigaChat tool provider (GigaChat-2-Pro by default) converts the agent's
OpenAI-style history to function_call / role=function; auto mode prefers GigaChat when credentials are set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_llm_agent.py

@@ -197,3 +197,75 @@ async def test_empty_final_answer_is_nudged(sertraline):
     assert ans is not None and "Алкоголь" in ans.text
     assert sent[0]["tool_choice"] == "required" and "tool_choice" not in sent[1]
     assert "сертралин" in sent[0]["messages"][0]["content"]  # patient brief in the system prompt
+
+
+async def test_region_block_string_error_degrades():
+    # Seen live from the RF server: 403 {"success": false, "error": "Access denied by security policy."}
+    client = httpx.AsyncClient(
+        transport=httpx.MockTransport(lambda r: httpx.Response(403, json={"success": False, "error": "Access denied"}))
+    )
+    provider = OpenRouterProvider(_settings(), client=client)
+    assert await agent.answer_chat(ANDREY, scrub("вопрос"), provider=provider, settings=_settings()) is None
+
+
+async def test_unexpected_provider_bug_degrades():
+    class Broken:
+        name = "broken"
+
+        async def chat(self, messages, tools):
+            raise AttributeError("boom")
+
+    assert await agent.answer_chat(ANDREY, scrub("вопрос"), provider=Broken(), settings=_settings()) is None
+
+
+# ------------------------------------------------------------------------------------------------- gigachat
+
+
+class _FakeGiga:
+    """Mimics gigachat.GigaChat.achat: first a function_call, then the final text."""
+
+    def __init__(self):
+        self.chats = []
+
+    async def achat(self, chat):
+        from types import SimpleNamespace as NS
+
+        self.chats.append(chat)
+        if len(self.chats) == 1:
+            fc = NS(name="check_drug_for_patient", arguments={"drug": "ибупрофен"})
+            msg = NS(content="", function_call=fc)
+        else:
+            msg = NS(
+                content="Ибупрофен вместе с сертралином повышает риск кровотечений. Обсудите с врачом.",
+                function_call=None,
+            )
+        return NS(choices=[NS(message=msg)], usage=NS(total_tokens=42))
+
+
+async def test_gigachat_tool_loop(sertraline):
+    from app.llm.agent.providers import GigaChatToolProvider
+
+    fake = _FakeGiga()
+    s = Settings(_env_file=None, chat_agent="gigachat", gigachat_credentials="x", chat_agent_timeout_s=5.0)
+    ans = await agent.answer_chat(
+        ANDREY, scrub("Можно ибупрофен?"), provider=GigaChatToolProvider(s, client=fake), settings=s
+    )
+    assert ans is not None and "кровотеч" in ans.text and ans.provider == "gigachat"
+    second = fake.chats[1]
+    roles = [str(m.role.value if hasattr(m.role, "value") else m.role) for m in second.messages]
+    assert roles == ["system", "user", "assistant", "function"]
+    assert second.messages[2].function_call.name == "check_drug_for_patient"
+    assert second.messages[3].name == "check_drug_for_patient"
+    assert {f.name for f in second.functions} == {
+        "search_guidelines",
+        "get_patient_record",
+        "get_drug_info",
+        "check_drug_for_patient",
+    }
+
+
+def test_build_provider_prefers_gigachat():
+    from app.llm.agent.providers import GigaChatToolProvider
+
+    s = Settings(_env_file=None, chat_agent="auto", gigachat_credentials="x", openrouter_api_key="y")
+    assert isinstance(agent.build_provider(s), GigaChatToolProvider)