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/database/queries/ai_providers.sql

@@ -66,6 +66,7 @@ RETURNING
 UPDATE
     ai_providers
 SET
+    type = @type::ai_provider_type,
     display_name = sqlc.narg('display_name')::text,
     enabled = @enabled::boolean,
     base_url = @base_url::text,