feat(agent-chat): 035-agent-chat-context — контекст, guardrails, tools, database discovery

== User stories ==
US1: Контекст с дашборда/датасета → /agent с URL params
US2: Guardrails card — env badge, 7 risk tones, countdown, permission_denied
US3: Tools optimization — retry, timeout, summarise, RBAC + context affinity

== Backend ==
- _context.py (NEW): UIContext validation (7 checks)
- _tool_filter.py (NEW): RBAC + context affinity pipeline
- _confirmation.py: build_confirmation_contract_v2, permission_denied_payload
- tools.py: superset_list_databases, retry/summarise/timeout wrappers
- app.py: _inject_uicontext, _inject_env_id_into_tools, database
  prefetch в runtime context
- _persistence.py: prefetch_databases()
- agent_superset_explore.py: GET /databases endpoint
- _llm_async_http.py, _persistence.py: fix double /v1 в LLM URL
  (LM Studio Unexpected endpoint)

== Frontend ==
- AgentChatModel.svelte.ts: 5 atoms, 3 actions, countdown, context
- AgentChat.svelte: production banner, process steps, debug panel
- ConfirmationCard.svelte: 7 risk tones, permission_denied, countdown
- ToolCallCard.svelte: retrying/timeout/cancelled states
- StreamProcessor.svelte.ts: tool_retry, timeout, permission_denied
- TopNavbar: sparkles icon + Ассистент
- sidebarNavigation: AI section
- DashboardHeader, datasets/+page: contextual AI buttons
- Icon: sparkles, brain, cpu icons
- tailwind: assistant category colors
- i18n: en/ru nav keys

== Tests ==
- 159 backend agent tests (+16 US3: retry, timeout, summarise, contracts)
- 2544 frontend tests (+11 model + component tests)
- 15 JSON fixtures (10 API + 5 model)

== Specs ==
- specs/035-agent-chat-context/: spec, UX, plan, tasks, research,
  data-model, contracts, quickstart, traceability, fixtures, checklists

Closes #035

backend/tests/test_agent/test_agent_handler.py

@@ -2,14 +2,15 @@
 # @BRIEF Tests for the Gradio agent handler — streaming, cancel, LLM error, empty message.
 # @RELATION BINDS_TO -> [AgentChat.GradioApp.Handler]
 import os
-import sys
 from pathlib import Path
+import sys
 
 sys.path.append(str(Path(__file__).parent.parent.parent / "src"))
 
-import jwt
 import pytest
-from unittest.mock import AsyncMock, patch, MagicMock
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import jwt
 
 # Set JWT_SECRET and LLM_API_KEY for tests
 JWT_SECRET = "test-jwt-secret-key"
@@ -35,6 +36,12 @@ def _make_test_jwt(user_id: str = "test-user") -> str:
     return jwt.encode({"sub": user_id}, JWT_SECRET, algorithm="HS256")
 
 
+def _empty_agent_state():
+    state = MagicMock(spec_set=["next"])
+    state.next = ()
+    return state
+
+
 # #region TestAgentChat.Handler.EmptyMessage [C:2] [TYPE Function] [SEMANTICS test,handler,empty]
 # @BRIEF Empty message returns immediately without calling LangGraph.
 # @TEST_EDGE empty_text, empty_with_files_none
@@ -50,7 +57,7 @@ async def test_handler_empty_message_returns_immediately():
     mock_request.client.host = "127.0.0.1"
 
     # Patch create_agent to avoid OpenAI init
-    with patch("src.agent.langgraph_setup.create_agent") as mock_create:
+    with patch("src.agent.langgraph_setup.create_agent"):
         # Empty message
         message = {"text": "", "files": None}
         results = []
@@ -85,11 +92,12 @@ async def test_handler_missing_auth_continues_gracefully():
     # Patch create_agent to prevent LLM call — handler should proceed without JWT
     with patch("src.agent.app.create_agent") as mock_create:
         mock_graph = AsyncMock()
-        async def _empty_stream(*args, **kwargs):
+        async def _empty_stream(*_args, **_kwargs):
             """Empty async generator — yields nothing."""
             return
             yield  # make it a generator
         mock_graph.astream_events = _empty_stream
+        mock_graph.aget_state = AsyncMock(return_value=_empty_agent_state())
         mock_create.return_value = mock_graph
 
         results = []
@@ -119,10 +127,11 @@ async def test_handler_invalid_jwt_continues_gracefully():
 
     with patch("src.agent.app.create_agent") as mock_create:
         mock_graph = AsyncMock()
-        async def _empty_stream(*args, **kwargs):
+        async def _empty_stream(*_args, **_kwargs):
             return
             yield
         mock_graph.astream_events = _empty_stream
+        mock_graph.aget_state = AsyncMock(return_value=_empty_agent_state())
         mock_create.return_value = mock_graph
 
         results = []
@@ -157,11 +166,12 @@ async def test_handler_yields_stream_tokens():
     with patch("src.agent.app.create_agent") as mock_create:
         mock_graph = AsyncMock()
 
-        async def _mock_stream(*args, **kwargs):
+        async def _mock_stream(*_args, **_kwargs):
             yield {"event": "on_chat_model_stream", "data": {"chunk": MagicMock(content="Hello")}}
             yield {"event": "on_chat_model_stream", "data": {"chunk": MagicMock(content=" world")}}
 
         mock_graph.astream_events = _mock_stream
+        mock_graph.aget_state = AsyncMock(return_value=_empty_agent_state())
         mock_create.return_value = mock_graph
 
         results = []
@@ -197,7 +207,7 @@ async def test_handler_resume_confirm():
     # imports create_agent directly from langgraph_setup
     with patch("src.agent._confirmation.create_agent") as mock_create:
         mock_graph = MagicMock()
-        async def _empty_stream(*args, **kwargs):
+        async def _empty_stream(*_args, **_kwargs):
             return
             yield
         mock_graph.astream_events = _empty_stream