[Bug]: websearch enterprise setting defaults to false and cannot be changed

### What happened?

When creating a websearch engine, if the enterprise setting is not explicitly specified, it defaults to false. I believe this is a bug. There is no way for applications to change the enterprise setting.
Expected behavior: Either make the default configurable or provide an API/interface for applications to change this setting.
Reproduction steps: When creating a new websearch engine, the enterprise setting is observed to be fixed to false. Attempts to change it from the application side fail because there is no settings UI.

```
/**
* Google Cloud Discovery Engine - Bug Reproduction Code (Without Enterprise Edition Setting)
*
* Issue: When searchTier is not set during engine creation, the engine is created as Standard Edition,
* but when trying to use Enterprise Edition features (extractiveContentSpec, returnSnippet),
* an error occurs. However, the behavior when searchTier is not explicitly set during engine creation
* may be inconsistent.
*/

import { DataStoreServiceClient, EngineServiceClient, protos } from '@google-cloud/discoveryengine'
import { GoogleAuth } from 'google-auth-library'

// Configuration
const PROJECT_ID = 'your-project-id'
const LOCATION = 'global'
const COLLECTION_ID = 'default_collection'
const DATA_STORE_ID = 'test-datastore-bug-reproduction'
const ENGINE_ID = 'test-engine-bug-reproduction'

async function reproduceBug() {
const auth = new GoogleAuth({
scopes: 'https://www.googleapis.com/auth/cloud-platform',
})

const client = await auth.getClient()
const credentials = await auth.getCredentials()

const dataStoreClient = new DataStoreServiceClient({
credentials: credentials as any,
})

const engineClient = new EngineServiceClient({
credentials: credentials as any,
})

const parentCollection = `projects/${PROJECT_ID}/locations/${LOCATION}/collections/${COLLECTION_ID}`
const parentDataStores = `${parentCollection}/dataStores`
const dataStoreName = `${parentDataStores}/${DATA_STORE_ID}`
const engineName = `${parentCollection}/engines/${ENGINE_ID}`

try {
// Step 1: Create DataStore
try {
const [op] = await dataStoreClient.createDataStore({
dataStore: {
contentConfig:
protos.google.cloud.discoveryengine.v1.DataStore.ContentConfig.PUBLIC_WEBSITE,
displayName: 'Bug Reproduction DataStore',
industryVertical: protos.google.cloud.discoveryengine.v1.IndustryVertical.GENERIC,
solutionTypes: [protos.google.cloud.discoveryengine.v1.SolutionType.SOLUTION_TYPE_SEARCH],
},
dataStoreId: DATA_STORE_ID,
parent: parentCollection,
})
await op.promise()
} catch (error: any) {
if (error.code !== 6) {
throw error
}
}

// Step 2: Create Engine without searchTier (should default to Standard Edition)
try {
const [op] = await engineClient.createEngine({
engine: {
dataStoreIds: [DATA_STORE_ID],
displayName: 'Bug Reproduction Engine (No Enterprise Setting)',
// Do not explicitly set searchTier (default should be Standard Edition)
searchEngineConfig: {},
solutionType: protos.google.cloud.discoveryengine.v1.SolutionType.SOLUTION_TYPE_SEARCH,
},
engineId: ENGINE_ID,
parent: parentCollection,
})
await op.promise()
} catch (error: any) {
if (error.code !== 6) {
throw error
}
}

// Step 3: Verify Engine Configuration
const [engine] = await engineClient.getEngine({ name: engineName })
const searchTier = engine.searchEngineConfig?.searchTier
console.log('Engine searchTier:', searchTier)
console.log('Expected: SEARCH_TIER_STANDARD (1) or SEARCH_TIER_UNSPECIFIED (0)')

// Step 4: Execute Search with Enterprise Edition Features
const token = await client.getAccessToken()

// Request including Enterprise Edition features
const searchRequest = {
contentSearchSpec: {
extractiveContentSpec: {
maxExtractiveAnswerCount: 1,
},
snippetSpec: {
returnSnippet: true,
},
},
pageSize: 10,
query: 'test query',
queryExpansionSpec: {
condition: 'AUTO',
},
spellCorrectionSpec: {
mode: 'AUTO',
},
}

const searchUrl = `https://discoveryengine.googleapis.com/v1alpha/projects/${PROJECT_ID}/locations/${LOCATION}/collections/${COLLECTION_ID}/engines/${ENGINE_ID}/servingConfigs/default_search:search`

const response = await fetch(searchUrl, {
body: JSON.stringify(searchRequest),
headers: {
Authorization: `Bearer ${token.token}`,
'Content-Type': 'application/json',
},
method: 'POST',
})

const responseText = await response.text()

if (response.ok) {
console.log('\n=== Unexpected Behavior ===')
console.log(
'Enterprise Edition features were usable on an engine created as Standard Edition.',
)
} else {
const errorData = JSON.parse(responseText)
console.log('\n=== Error Occurred ===')
console.log('Error code:', errorData.error?.code)
console.log('Error message:', errorData.error?.message)

if (
errorData.error?.message?.includes('enterprise edition features') ||
errorData.error?.message?.includes('standard edition')
) {
console.log('\n=== Bug Confirmed ===')
console.log('Issue: When searchTier is not set, the engine is created as Standard Edition,')
console.log('but when trying to use Enterprise Edition features, an error occurs.')
}
}
} catch (error) {
console.error('Error during bug reproduction:', error)

throw error
}
}

// Execute
try {
await reproduceBug()
} catch (error) {
console.error('Failed to reproduce bug:', error)
process.exit(1)
}
```

### Relevant log output

```shell
Error: Vertex AI Search: Search API request failed: Vertex AI Search: Search API request failed: HTTP error! status: 400, response: {
[chatbot] "error": {
[chatbot] "code": 400,
[chatbot] "message": "Cannot use enterprise edition features (website search, multi-modal search, extractive answers/segments, etc.) in a standard edition search engine. Please follow https://cloud.google.com/generative-ai-app-builder/docs/enterprise-edition#toggle-enterprise to enable Enterprise edition. Enterprise edition can be only enabled at the engine / app level. If the search request is against a data store, please update the serving config in the search request to use the engine/app ID instead, in the format of: \"projects/*/locations/*/collections/*/engines/*/servingConfigs/*\".",
[chatbot] "status": "FAILED_PRECONDITION"
[chatbot] }
[chatbot] }
```

### Code of Conduct

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