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 --
```

Open google/cloud/universe_domain/integration_tests/impersonation_tests.cc and inspect the ExternalAccountImpersonationTest.EAToSAImpersonationRest and EAToSAImpersonationGrpc tests around the reported failure. Check how integration tests are disabled, then run the relevant test target to confirm both failing tests are excluded and the remaining suite passes.

Scritto dal modello di indicizzazione a partire dal testo della issue.