Fix flaky GCS ADC fallback test by isolating in its own JVM

[jdconrad]

• Move testApplicationDefaultCredentialsFallbackToComputeEngine from GoogleCloudStorageServiceTests to a new GoogleCloudStorageAdcTests class
• Run it via a dedicated gcsAdcTest Gradle task in its own JVM, ensuring the DefaultCredentialsProvider singleton starts with clean state (cachedCredentials, checkedComputeEngine)
• Add auth validation and metadata request handling to the GCS test fixture

The test was failing ~38% of the time (#145689) because Google's DefaultCredentialsProvider singleton caches credentials and GCE detection state. When another test in the same JVM primed the singleton before this test ran, the credential discovery flow (including the File.isFile() entitlement check) was bypassed entirely.

Closes #145689

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository YAML (base), Organization UI (inherited)

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Run ID: 3bd7be9d-c3c7-438f-a4df-142b0186022b

📥 Commits

Reviewing files that changed from the base of the PR and between 44a079d and 337f119.

📒 Files selected for processing (2)

• modules/repository-gcs/build.gradle
• modules/repository-gcs/src/test/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageAdcTests.java

🚧 Files skipped from review as they are similar to previous changes (2)

• modules/repository-gcs/src/test/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageAdcTests.java
• modules/repository-gcs/build.gradle

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📝 Walkthrough

Walkthrough

This change addresses test flakiness by restructuring how the Application Default Credentials fallback test executes. The test method testApplicationDefaultCredentialsFallbackToComputeEngine() was moved from GoogleCloudStorageServiceTests to a new dedicated test class GoogleCloudStorageAdcTests. Correspondingly, the Gradle build configuration was updated to create an isolated gcsAdcTest task that runs only this test while explicitly removing the GOOGLE_APPLICATION_CREDENTIALS environment variable. The default test task excludes this test, and the check task now depends on both the existing gcsThirdPartyTest and the new gcsAdcTest.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	PR successfully isolates the flaky ADC test into a dedicated JVM via gcsAdcTest task, directly addressing issue #145689's root cause (DefaultCredentialsProvider singleton caching).
Out of Scope Changes check	✅ Passed	All changes directly support the objective: test relocation, Gradle task isolation, and removal of the original test from GoogleCloudStorageServiceTests.

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✨ Finishing Touches

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• Create PR with unit tests

• 🛠️ Update Documentation: Commit on current branch
• 🛠️ Update Documentation: Create PR

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