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/models.go

@@ -5119,7 +5119,6 @@ type ChatMessage struct {
 	RuntimeMs           sql.NullInt64         `db:"runtime_ms" json:"runtime_ms"`
 	Deleted             bool                  `db:"deleted" json:"deleted"`
 	ProviderResponseID  sql.NullString        `db:"provider_response_id" json:"provider_response_id"`
-	APIKeyID            sql.NullString        `db:"api_key_id" json:"api_key_id"`
 	Revision            int64                 `db:"revision" json:"revision"`
 	// Stores the selected effort for the turn triggered by this message.
 	ReasoningEffort NullChatReasoningEffort `db:"reasoning_effort" json:"reasoning_effort"`
@@ -5151,7 +5150,6 @@ type ChatQueuedMessage struct {
 	Content       json.RawMessage `db:"content" json:"content"`
 	CreatedAt     time.Time       `db:"created_at" json:"created_at"`
 	ModelConfigID uuid.NullUUID   `db:"model_config_id" json:"model_config_id"`
-	APIKeyID      sql.NullString  `db:"api_key_id" json:"api_key_id"`
 	Position      int64           `db:"position" json:"position"`
 	CreatedBy     uuid.UUID       `db:"created_by" json:"created_by"`
 	// Stores the selected effort until the queued row is promoted.