feat(bot): chat agent on Gemini 2.5 Flash-Lite — grounded first turn, current-prescription defaults

Live-tested on OpenRouter: Flash-Lite answers in 3–8 s with correct findings at ~$0.10/$0.40 per 1M tokens
(qwen3.7-flash and gpt-oss-120b took 26–32 s). Force a tool call on the first turn, give the model the
current prescription, let check_drug_for_patient default to it and return composition, intolerance check
and general interactions (alcohol); retry once on an empty final message.

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

tests/test_llm_agent.py

@@ -169,3 +169,31 @@ def test_build_provider_modes():
     assert isinstance(agent.build_provider(Settings(_env_file=None, chat_agent="mock")), MockAgentProvider)
     assert agent.build_provider(Settings(_env_file=None, chat_agent="openrouter")) is None  # no key
     assert agent.build_provider(Settings(_env_file=None, chat_agent="off")) is None
+
+
+async def test_check_defaults_to_current_prescription(sertraline):
+    out = json.loads(await run_tool(ToolContext(token=ANDREY), "check_drug_for_patient", {"drug": "мои таблетки"}))
+    assert out["found"] and out["inn"] == "сертралин"
+    assert "лактоз" in out["intolerance_check"] and "нет" in out["intolerance_check"]
+    assert any("Алкоголь" in t for t in out["general_interactions"])
+
+
+async def test_empty_final_answer_is_nudged(sertraline):
+    replies = iter(
+        [
+            {"role": "assistant", "content": None, "tool_calls": _CALLS},
+            {"role": "assistant", "content": ""},
+            {"role": "assistant", "content": "Алкоголь во время лечения не рекомендуется. Обсудите с врачом."},
+        ]
+    )
+    sent = []
+
+    def handler(request):
+        sent.append(json.loads(request.content))
+        return httpx.Response(200, json={"choices": [{"message": next(replies)}]})
+
+    provider = OpenRouterProvider(_settings(), client=httpx.AsyncClient(transport=httpx.MockTransport(handler)))
+    ans = await agent.answer_chat(ANDREY, scrub("пиво можно?"), provider=provider, settings=_settings())
+    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