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/workflow_graph.py

@@ -89,6 +89,7 @@ class Node:
         self.delayed_start_duration = getattr(data, "delayed_start_duration", None)
         self.tool_uuids = getattr(data, "tool_uuids", None)
         self.document_uuids = getattr(data, "document_uuids", None)
+        self.mcp_tool_filters = getattr(data, "mcp_tool_filters", None)
         self.pre_call_fetch_enabled = getattr(data, "pre_call_fetch_enabled", False)
         self.pre_call_fetch_url = getattr(data, "pre_call_fetch_url", None)
         self.pre_call_fetch_credential_uuid = getattr(