Adds a database migration that reconciles existing stale chat MCP server
IDs, then installs a `BEFORE DELETE` trigger on `mcp_server_configs` to
remove the deleted ID from `chats.mcp_server_ids`. This keeps chat
continuation from failing with `400 One or more MCP server IDs are
invalid` after an MCP server config is deleted.
This matches the existing repo precedent in
`coderd/database/migrations/000241_delete_user_roles.up.sql`, where
deleting a custom role cleans `organization_members.roles`, a similarly
structured array of references that cannot be protected by a normal
foreign key.
Closes CODAGT-505
Ethan
eb2c2799ca