fix: restore isAppConfigSpecification filter in createConfigExtensionInstances

The rebase replaced isAppConfigSpecification(spec) with
uidStrategy === 'single', which is NOT equivalent — uidStrategy 'single'
includes webhook subscriptions (experience: 'extension'), while
isAppConfigSpecification only matches experience: 'configuration'.

Also restored the missing WebhookSubscriptionSpecIdentifier exclusion
filter that the rebase dropped.

Without this fix, config extensions aren't properly identified during
deploy, causing the API to reject with "at least one specification
file is required".

Verified: all 12 E2E tests pass locally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ryancbahan

packages/app/src/cli/models/app/loader.ts

@@ -22,7 +22,7 @@ import {configurationFileNames, dotEnvFileNames} from '../../constants.js'
 import metadata from '../../metadata.js'
 import {ExtensionInstance} from '../extensions/extension-instance.js'
 import {ExtensionsArraySchema, UnifiedSchema} from '../extensions/schemas.js'
-import {ExtensionSpecification} from '../extensions/specification.js'
+import {ExtensionSpecification, isAppConfigSpecification} from '../extensions/specification.js'
 import {CreateAppOptions, Flag} from '../../utilities/developer-platform-client.js'
 import {findConfigFiles} from '../../prompts/config.js'
 import {WebhookSubscriptionSpecIdentifier} from '../extensions/specifications/app_config_webhook_subscription.js'
@@ -763,7 +763,8 @@ class AppLoader<TConfig extends CurrentAppConfiguration, TModuleSpec extends Ext
     const configPath = this.loadedConfiguration.configPath
     const extensionInstancesWithKeys = await Promise.all(
       this.specifications
-        .filter((specification) => specification.uidStrategy === 'single')
+        .filter((specification) => isAppConfigSpecification(specification))
+        .filter((specification) => specification.identifier !== WebhookSubscriptionSpecIdentifier)
         .map(async (specification) => {
           const specConfiguration = parseConfigurationObjectAgainstSpecification(
             specification,