Re-enable EAToSAImpersonationRest and EAToSAImpersonationGrpc

Currently we see the error, we need to disable it now. I believe it is a temp error, we can try to re-enable it later.

```
Step #2: [ RUN ] ExternalAccountImpersonationTest.EAToSAImpersonationRest
Step #2: google/cloud/universe_domain/integration_tests/impersonation_tests.cc:159: Failure
Step #2: Value of: disk
Step #2: Expected: code is equal to OK and message is anything
Step #2: Actual: 608-byte object , whose status is INVALID_ARGUMENT: Permanent error, with a last message of {"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."} error_info={reason=, domain=, metadata={gcloud-cpp.retry.function=ListDisks, gcloud-cpp.retry.reason=permanent-error, gcloud-cpp.retry.original-message={"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}}}, with a code that isn't equal to OK, but a message that is anything
Step #2:
Step #2: [ FAILED ] ExternalAccountImpersonationTest.EAToSAImpersonationRest (152 ms)
Step #2: [ RUN ] ExternalAccountImpersonationTest.EAToSAImpersonationGrpc
Step #2: WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
Step #2: E0000 00:00:1753797582.907180 38 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797584.895745 30 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797587.633246 30 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797595.205897 27 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797603.271555 37 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797625.157134 34 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797687.287670 34 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797716.359216 39 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753797774.126996 30 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753798071.531197 28 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: E0000 00:00:1753798325.601230 31 external_account_credentials.cc:395] Fetch external account credentials access token: UNAVAILABLE: Call to HTTP server ended with status 400 [{"error":"invalid_grant","error_description":"Error connecting to the given credential's issuer."}]
Step #2: -- Test timed out at 2025-07-29 14:14:39 UTC --
```

google/cloud/universe_domain/integration_tests/impersonation_tests.cc を開き、報告された失敗に関連する ExternalAccountImpersonationTest.EAToSAImpersonationRest テストと EAToSAImpersonationGrpc テストを調査します。統合テストを無効化する方法を確認してから、関連するテストターゲットを実行し、失敗している両方のテストが除外され、残りのテストスイートがパスすることを確認します。

索引モデルが issue の本文から書いたものです。