fix: backfill legacy Bedrock AI provider rows and stale model config strings (#26155)

Fixes CODAGT-548

Adds two idempotent startup backfills run after `newAPI():

- `BackfillBedrockProviderType`: promotes `ai_providers` rows from
`type=anthropic` with Bedrock settings to `type=bedrock`.
- `BackfillChatModelConfigProviderStrings`: fixes stale
`chat_model_configs.provider = "anthropic"` strings on rows whose linked
provider was just promoted.
- `UpdateAIProvider` query now also writes the `type` column, so the
fix persists on any subsequent PATCH.


> 🤖 Generated by Claude with oversight from a human.

coderd/x/chatd/chatd_test.go

@@ -6310,6 +6310,7 @@ func setOpenAIProviderBaseURL(
 		}
 		_, err = db.UpdateAIProvider(ctx, database.UpdateAIProviderParams{
 			ID:            provider.ID,
+			Type:          provider.Type,
 			DisplayName:   provider.DisplayName,
 			Enabled:       provider.Enabled,
 			BaseUrl:       baseURL,