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/ai_providers.go

@@ -351,6 +351,7 @@ func (api *API) aiProvidersUpdate(rw http.ResponseWriter, r *http.Request) {
 		}
 		params := database.UpdateAIProviderParams{
 			ID:          old.ID,
+			Type:        old.Type,
 			DisplayName: displayName,
 			Enabled:     ptr.NilToDefault(req.Enabled, old.Enabled),
 			BaseUrl:     ptr.NilToDefault(req.BaseURL, old.BaseUrl),