feat: MCP Client plugin (Task 5.17) - integrate external MCP servers for AI agents

- New plugin app/plugins/builtins/mcp_client/ with server config CRUD
- Models: McpServerConfig with TenantMixin (name, url, api_token, enabled)
- Routes: GET/POST/PATCH/DELETE /api/v1/mcp-client/servers
- Routes: GET /servers/{id}/tools, POST /servers/{id}/execute
- client.py: async MCP client using httpx for external server calls
- tool_registry_integration.py: registers external MCP tools in AI tool registry
- Migration: 0001_initial.sql for mcp_server_configs table
- Frontend: mcpClient.ts API client with React Query hooks
- Frontend: MCP Client settings UI in SettingsMcp.tsx (server CRUD, tool viewing)
- Tests: 8 tests covering CRUD, auth, tool registry integration

app/plugins/builtins/mcp_client/schemas.py

@@ -0,0 +1,69 @@
+"""Pydantic schemas for MCP Client plugin."""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class McpServerConfigCreate(BaseModel):
+    """Create a new MCP server configuration."""
+    name: str = Field(..., min_length=1, max_length=200)
+    url: str = Field(..., min_length=1, max_length=500)
+    api_token: str | None = None
+    enabled: bool = True
+    description: str | None = None
+
+
+class McpServerConfigUpdate(BaseModel):
+    """Update an existing MCP server configuration."""
+    name: str | None = Field(None, min_length=1, max_length=200)
+    url: str | None = Field(None, min_length=1, max_length=500)
+    api_token: str | None = None
+    enabled: bool | None = None
+    description: str | None = None
+
+
+class McpServerConfigResponse(BaseModel):
+    """Response model for MCP server configuration."""
+    id: str
+    name: str
+    url: str
+    api_token: str | None = None
+    enabled: bool
+    description: str | None = None
+    last_connected_at: datetime | None = None
+    created_by: str | None = None
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class McpServerToolInfo(BaseModel):
+    """Tool info from an external MCP server."""
+    name: str
+    description: str = ""
+    parameters: dict = Field(default_factory=dict)
+
+
+class McpServerToolsResponse(BaseModel):
+    """Response listing tools from an external MCP server."""
+    server_name: str
+    server_url: str
+    tools: list[McpServerToolInfo]
+    count: int
+
+
+class McpServerExecuteRequest(BaseModel):
+    """Request to execute a tool on an external MCP server."""
+    tool_name: str
+    arguments: dict = Field(default_factory=dict)
+
+
+class McpServerExecuteResponse(BaseModel):
+    """Response from executing a tool on an external MCP server."""
+    server_name: str
+    tool: str
+    success: bool
+    result: dict | str | None = None
+    error: str | None = None