feat(agent-superset): extend SupersetClient with agent-critical methods + DDL/DML guard + tests (126 passed)

Ported from mcp-superset research module, integrated into existing async SupersetClient:

New mixins (4 files):
- safety.py: DDL/DML guard (13 keywords), comment/string stripping, viz_type validation
- _sql_lab.py: execute_sql, format_sql, results, estimate, CSV export, query history
- _saved_queries.py: saved queries CRUD (5 methods)
- _audit.py: permissions audit matrix (user × dashboard × dataset × RLS)

Extended mixins (4 files):
- _dashboards_write.py: +create, +update (general), +copy, +publish, +unpublish
- _dashboards_crud.py: +standalone get_dashboard_charts/datasets, +get_dashboard
- _datasets.py: +create, +delete, +duplicate, +refresh_schema, +get_or_create, +export/import
- _databases.py: +update, +test_connection, +schemas/tables/catalogs, +validate_sql, +select_star, +table_metadata

FastAPI proxy (2 files):
- agent_superset.py (284L): SQL Lab + Dashboards/Datasets write endpoints
- agent_superset_explore.py (240L): DB explore + Audit + Saved Queries endpoints

Agent tools (2 files):
- tools.py: +7 LangChain @tools (24 total), intent-routing keywords
- _tool_resolver.py: updated SAFE/GUARDED/FAST_CONFIRM classification sets

Tests (3 files, 126 tests):
- test_superset_safety.py (51): DDL/DML bypass/legitimate/safe scenarios
- test_superset_extended.py (42): all mixin methods with mock AsyncAPIClient
- test_superset_tools.py (33): agent tool registration, intent matching, @tool .ainvoke()

backend/src/agent/_tool_resolver.py

@@ -23,6 +23,11 @@ _SAFE_AGENT_TOOLS = {
     "list_llm_providers",
     "get_llm_status",
     "list_maintenance_events",
+    # NEW: read-only Superset tools
+    "superset_execute_sql",
+    "superset_explore_database",
+    "superset_audit_permissions",
+    "superset_format_sql",
 }
 _GUARDED_AGENT_TOOLS = {
     "create_branch",
@@ -33,6 +38,10 @@ _GUARDED_AGENT_TOOLS = {
     "run_llm_documentation",
     "start_maintenance",
     "end_maintenance",
+    # NEW: guarded Superset write operations
+    "superset_create_dashboard",
+    "superset_copy_dashboard",
+    "superset_create_dataset",
 }
 _DANGEROUS_AGENT_TOOLS = {
     "deploy_dashboard",
@@ -44,6 +53,9 @@ _FAST_CONFIRM_TOOLS = {
     "list_llm_providers",
     "get_llm_status",
     "list_maintenance_events",
+    "superset_explore_database",
+    "superset_audit_permissions",
+    "superset_format_sql",
 }
 # #endregion AgentChat.ToolResolver.Sets