Hold a deleted CoderControlPlane while CoderMCPServer objects still use it #382

Problem

When a CoderControlPlane is deleted, the hold from #362 makes it wait for its CoderAIGatewayKey and CoderAIProvider objects. It does not wait for CoderMCPServer objects.

controlplane_hold.go lists only those two kinds in controlPlaneDependents. If the control plane is deleted first, the CoderMCPServer delete path makes one delete attempt with a 10-second limit, or takes the ControlPlaneGone path. Both release the finalizer. The MCP server config can then stay in the Coder database, enabled and with its stored credential.

The CoderMCPServer how-to tells users to delete CoderMCPServer objects before their control plane. It does not say that the hold leaves MCP servers out.

Fix

1. Add CoderMCPServer to controlPlaneDependents and to requestForDependentControlPlane.
2. Add a test that the control plane waits for a CoderMCPServer.
3. Update the how-to's "Delete a server" section.

Refs #353, refs #155. Found in the final assessment of #381.

Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: medium