refactor: drop chat_model_configs provider column (#26877)

The provider type already lives authoritatively in ai_providers.type,
reachable on every active row through ai_provider_id, which the
chat_model_configs_ai_provider_required_when_active CHECK makes
mandatory. The stored provider string was a denormalized copy the system
kept in sync with a startup backfill and no longer needs.

Every surface now derives provider type from the linked ai_providers
row. Telemetry is the one exception: it keeps emitting provider, now
sourced from ai_providers.type via a JOIN, so the BigQuery column and the
Nexus dashboards that read it are unaffected. The experimental HTTP/SDK
response drops provider and makes ai_provider_id required, since those
endpoints return only active configs; consumers resolve provider type
from ai_provider_id and the AI providers listing.

This ships in a single release with no compatibility window: production
reads the table via SELECT *, so a pre-drop binary fails config reads the
moment the column is gone. Operators must scale to zero before upgrading,
and there is no rollback.

Closes CODAGT-599

coderd/database/models.go

@@ -4953,7 +4953,6 @@ type ChatMessage struct {
 
 type ChatModelConfig struct {
 	ID                   uuid.UUID       `db:"id" json:"id"`
-	Provider             string          `db:"provider" json:"provider"`
 	Model                string          `db:"model" json:"model"`
 	DisplayName          string          `db:"display_name" json:"display_name"`
 	CreatedBy            uuid.NullUUID   `db:"created_by" json:"created_by"`