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

cli/exp_scaletest_chat_test.go

@@ -128,7 +128,7 @@ func chatMessageText(messages []codersdk.ChatMessage, role codersdk.ChatMessageR
 func scaletestModelConfigsForProvider(configs []codersdk.ChatModelConfig, providerID uuid.UUID) []codersdk.ChatModelConfig {
 	matches := make([]codersdk.ChatModelConfig, 0, 1)
 	for _, config := range configs {
-		if config.AIProviderID == nil || *config.AIProviderID != providerID {
+		if config.AIProviderID != providerID {
 			continue
 		}
 		if config.Model != "scaletest-model" {

cli/server.go

@@ -1117,9 +1117,6 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 			}
 			// Must run after newAPI so options.Database is dbcrypt-wrapped.
 			coderd.BackfillBedrockProviderType(aibridgeInitCtx, options.Database, logger.Named("aibridge.backfill"))
-			// Must run after BackfillBedrockProviderType; shares aibridgeInitCtx so
-			// a timeout on the first backfill will skip this one until next startup.
-			coderd.BackfillChatModelConfigProviderStrings(aibridgeInitCtx, options.Database, logger.Named("aibridge.backfill"))
 
 			// In-memory aibridge daemon. Registered on coderd so chatd can
 			// dispatch LLM requests via the in-process transport without