fix(coderd): prevent stale chat model config writes (#27968)

Chat model updates could merge PATCH fields from a row read before the
serialized transaction. Deletes could also succeed after another request
deleted the row.

Merge updates from the row read after the write lock. Make soft deletes
report missing rows, and validate provider availability and model
compatibility in the same transaction.

This PR also removes `DeleteChatModelConfigsByAIProviderID`. The query
has no production callers, including the AI provider deletion path, and
only remained in generated database wrappers and mocks. Its removal is
intentional dead-code cleanup and does not change provider deletion
behavior.

_This pull request description was generated by Coder Agents._

coderd/database/queries.sql.go

@@ -6135,7 +6135,7 @@ func (q *sqlQuerier) InsertChatFile(ctx context.Context, arg InsertChatFileParam
 	return i, err
 }
 
-const deleteChatModelConfigByID = `-- name: DeleteChatModelConfigByID :exec
+const deleteChatModelConfigByID = `-- name: DeleteChatModelConfigByID :one
 UPDATE
     chat_model_configs
 SET
@@ -6144,28 +6144,15 @@ SET
     updated_at = NOW()
 WHERE
     id = $1::uuid
-`
-
-func (q *sqlQuerier) DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) error {
-	_, err := q.db.ExecContext(ctx, deleteChatModelConfigByID, id)
-	return err
-}
-
-const deleteChatModelConfigsByAIProviderID = `-- name: DeleteChatModelConfigsByAIProviderID :exec
-UPDATE
-    chat_model_configs
-SET
-    deleted = TRUE,
-    deleted_at = NOW(),
-    updated_at = NOW()
-WHERE
-    ai_provider_id = $1::uuid
     AND deleted = FALSE
+RETURNING id
 `
 
-func (q *sqlQuerier) DeleteChatModelConfigsByAIProviderID(ctx context.Context, aiProviderID uuid.UUID) error {
-	_, err := q.db.ExecContext(ctx, deleteChatModelConfigsByAIProviderID, aiProviderID)
-	return err
+func (q *sqlQuerier) DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) (uuid.UUID, error) {
+	row := q.db.QueryRowContext(ctx, deleteChatModelConfigByID, id)
+	var id_2 uuid.UUID
+	err := row.Scan(&id_2)
+	return id_2, err
 }
 
 const getChatModelConfigByID = `-- name: GetChatModelConfigByID :one