Add MCP server disconnect functionality

What does this MR do and why?

Related issue: Enable custom agents to access external data vi... (#590708)

Add MCP server disconnect functionality

• Add RevokeTokenService that attempts provider token revocation (RFC 7009) before destroying the local McpServersUser record; returns an error if destroy fails, ignores provider-side errors
• Add disconnect action and POST route to McpConnectorsAuthController, returning JSON 200/422
• Expose currentUserConnected boolean field on AiCatalogMcpServer GraphQL type via connected_by_user? model method
• Show Disconnect button in place of Connect when the current user is already connected; handle disconnect with axios POST + Apollo refetch
• Add disconnectMcpServer utility alongside connectMcpServer in utils.js
• Add full spec coverage: RevokeTokenService (12 examples), disconnect action in controller spec (6 examples), type spec field check

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