refactor(coderd): stop storing chat gateway key IDs and drop the columns (#27171)

> Mux is working on behalf of Mike.

## Summary

Stop reading and writing the legacy `api_key_id` columns on chat
messages and queued messages, and drop the columns in the same PR.
Runtime AI Gateway attribution continues to use the per-user synthetic
key introduced by #27170.

With the columns gone, `sqlc` generates `database.ChatMessage` and
`database.ChatQueuedMessage` without `api_key_id`, so no transitional
query scaffolding is needed.

Migration `000548` drops the `api_key_id` columns. #27170 already
removed their foreign keys, so the down migration re-adds nullable text
columns without constraints. Previous column values cannot be restored.

Also moves the model config validation in `CreateChat` above the
message-building work so a disabled or invalid model fails fast. On main
this mattered more: the old ordering minted a synthetic API key before
rejecting the request.

Deploy note: replicas still running the previous release write
`api_key_id` on insert, so chat message inserts on old replicas fail
during the rolling window after the column drop. This was previously
split across two PRs to avoid that window; per review feedback the split
added more churn than it was worth for an experimental surface.

Depends on #27170 (merged).

coderd/database/migrations/000548_drop_chat_gateway_key_columns.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE chat_messages
+	ADD COLUMN api_key_id text;
+
+ALTER TABLE chat_queued_messages
+	ADD COLUMN api_key_id text;

coderd/database/migrations/000548_drop_chat_gateway_key_columns.up.sql

@@ -0,0 +1,5 @@
+ALTER TABLE chat_messages
+	DROP COLUMN api_key_id;
+
+ALTER TABLE chat_queued_messages
+	DROP COLUMN api_key_id;