feat: support generic user-configured MCP servers via Admin credentials

Motivation

Teams wanting to connect any additional MCP server to the agent today have no path without forking agent/integrations/ or opening a vendor-specific PR.

Proposal

Add a “Custom MCP server” entry to the Admin credentials UI alongside Datadog and LangSmith , everything else follows the existing pattern: encrypted at rest, loaded server-side via langchain-mcp-adapters (already a dep), same OBSERVABILITY_AUTHORIZED_EMAILS gate.

We are happy to contribute this if the approach sounds good.