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>

docs/docs.json

@@ -71,7 +71,8 @@
                   {
                     "group": "Custom Tools",
                     "pages": [
-                      "voice-agent/tools/http-api"
+                      "voice-agent/tools/http-api",
+                      "voice-agent/tools/mcp-tool"
                     ]
                   }
                 ]
@@ -308,4 +309,4 @@
       "linkedin": "https://linkedin.com/company/dograh"
     }
   }
-}
+}