fix: strip deleted MCP IDs from chats on delete (#25763)

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

coderd/database/migrations/000510_cleanup_chats_mcp_server_ids_on_delete.down.sql

@@ -0,0 +1,2 @@
+DROP TRIGGER IF EXISTS remove_chat_mcp_server_config_id ON mcp_server_configs;
+DROP FUNCTION IF EXISTS remove_mcp_server_config_id_from_chats;

coderd/database/migrations/000510_cleanup_chats_mcp_server_ids_on_delete.up.sql

@@ -0,0 +1,41 @@
+-- Remove already-stale MCP server references before future deletes are
+-- handled by the trigger below.
+UPDATE chats
+SET mcp_server_ids = (
+	SELECT COALESCE(array_agg(ids.mcp_server_id ORDER BY ids.position), '{}'::uuid[])
+	FROM unnest(chats.mcp_server_ids) WITH ORDINALITY AS ids(mcp_server_id, position)
+	WHERE EXISTS (
+		SELECT 1
+		FROM mcp_server_configs
+		WHERE mcp_server_configs.id = ids.mcp_server_id
+	)
+)
+WHERE EXISTS (
+	SELECT 1
+	FROM unnest(chats.mcp_server_ids) AS ids(mcp_server_id)
+	WHERE NOT EXISTS (
+		SELECT 1
+		FROM mcp_server_configs
+		WHERE mcp_server_configs.id = ids.mcp_server_id
+	)
+);
+
+CREATE OR REPLACE FUNCTION remove_mcp_server_config_id_from_chats()
+	RETURNS TRIGGER AS
+$$
+BEGIN
+	UPDATE chats
+	SET mcp_server_ids = array_remove(mcp_server_ids, OLD.id)
+	WHERE OLD.id = ANY(mcp_server_ids);
+	RETURN OLD;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER remove_chat_mcp_server_config_id
+	BEFORE DELETE ON mcp_server_configs FOR EACH ROW
+	EXECUTE PROCEDURE remove_mcp_server_config_id_from_chats();
+
+COMMENT ON TRIGGER
+	remove_chat_mcp_server_config_id
+	ON mcp_server_configs IS
+		'When an MCP server config is deleted, this trigger removes its ID from all chats.';