feat(mcp): generic MCP tool source with per-node function filtering (#301)

* feat(mcp): generic MCP tool source with per-node function filtering

Adds a Model Context Protocol tool category: connect a customer MCP
server and expose its tools to the agent, with optional per-node
allow-listing of individual MCP functions.

- ToolCategory.MCP enum + alembic migration
- MCP definition validator and collision-safe function-name namespacing
- McpToolSession wrapper: graceful-degrade, per-call open/close lifecycle
- CustomToolManager MCP branch (schemas + proxy handlers)
- Per-node mcp_tool_filters threaded through DTO/graph/engine
- Best-effort discovered_tools catalog cache + POST /tools/{uuid}/mcp/refresh
- UI: MCP create/edit config, tabbed ToolSelector with per-node toggles

* feat: refactor for code standardisation and documentation

---------

Co-authored-by: Abhishek Kumar <abhishek@a6k.me>

api/services/workflow/dto.py

@@ -1,5 +1,5 @@
 from enum import Enum
-from typing import Annotated, List, Literal, Optional, Union
+from typing import Annotated, Dict, List, Literal, Optional, Union
 
 from pydantic import BaseModel, Field, ValidationError, model_validator
 
@@ -69,6 +69,7 @@ class _ExtractionNodeDataMixin(BaseModel):
 class _ToolDocumentRefsMixin(BaseModel):
     tool_uuids: Optional[List[str]] = None
     document_uuids: Optional[List[str]] = None
+    mcp_tool_filters: Optional[Dict[str, List[str]]] = None
 
 
 class StartCallNodeData(