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

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

api/enums.py

@@ -133,6 +133,7 @@ class ToolCategory(Enum):
     CALCULATOR = "calculator"  # Built-in calculator tool
     NATIVE = "native"  # Built-in integrations (future: dtmf_input)
     INTEGRATION = "integration"  # Third-party integrations (future: Google Calendar, Salesforce, etc.)
+    MCP = "mcp"  # Customer-provided MCP server exposing a tool catalog
 
 
 class ToolStatus(Enum):