Add token caching for AzurePipelinesCredential

[Copilot]

• Analyze the root cause of GetAllImageDigestsFromRegistryAsync performance regression
• Implement CachingTokenCredential wrapper that caches tokens and refreshes them when near expiration
• Update AzureTokenCredentialProvider to wrap AzurePipelinesCredential with CachingTokenCredential
• Add unit tests for CachingTokenCredential
• Run code review and address feedback (fixed race condition using SemaphoreSlim, added documentation, improved nullable check)
• Fix build error: Add InternalsVisibleTo attribute to allow test project to access internal CachingTokenCredential class
• Enable nullable annotations in test file
• Run security scan (timed out - no issues expected as changes are straightforward token caching)

Root Cause Analysis

PR #1879 removed the StaticTokenCredential class that was caching tokens. The AzurePipelinesCredential was returned directly, but it does NOT cache tokens internally. Each call to GetToken fetches a new token from Azure, turning what should be a 1-minute operation into 1+ hours.

Solution

Implemented a CachingTokenCredential wrapper that:

1. Caches the token after the first fetch
2. Returns the cached token if it's still valid (not within 5 minutes of expiration)
3. Refreshes the token only when it's about to expire
4. Uses SemaphoreSlim to ensure thread-safe access and prevent concurrent token fetches

Original prompt

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This section details on the original issue you should resolve

<issue_title>GetAllImageDigestsFromRegistryAsync is suddenly extremely slow</issue_title>
<issue_description>In this dotnet-docker-nightly pipeline run it took 1hr15min, causing the pipeline to time out later in the run:

2025-12-19T01:01:15.0763844Z Querying registry for all image digests...
2025-12-19T02:15:43.0489446Z Checking digest for existing annotation: ...

Previously (in this pipeline run), it took less than one minute:

2025-12-10T21:56:14.9556032Z Querying registry for all image digests...
2025-12-10T22:19:31.3493702Z Checking digest for existing annotation: ...

The earliest occurrence of this behavior I could find is in https://dev.azure.com/dnceng/internal/_build/results?buildId=2861673&view=logs&j=739bb7ec-7296-5b40-7dff-ee3297bfdaa4&t=a08adfce-16dd-580a-541c-89ea6031d6c4 where it took 23 minutes. That is dotnet-docker commit db838f02ef8fb99154680b938054748dc856d8b9.

The cause is likely #1879.</issue_description>

<agent_instructions>Look through recent commits to diagnose the performance issue. Pay particular attention to PR #1879. We already know that manifests and repos are checked sequentially - that is known not to cause performance issues in this case. It is most likely something to do with the AzurePipelinesCredential/Token fetching since that code changed recently. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes GetAllImageDigestsFromRegistryAsync is suddenly extremely slow #1905

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[lbussell]

@copilot /repo/src/ImageBuilder.Tests/CachingTokenCredentialTests.cs(29,37): error CS0122: 'CachingTokenCredential' is inaccessible due to its protection level

[Copilot]

@copilot /repo/src/ImageBuilder.Tests/CachingTokenCredentialTests.cs(29,37): error CS0122: 'CachingTokenCredential' is inaccessible due to its protection level

Fixed in 8cfb114 by adding InternalsVisibleTo attribute to the ImageBuilder project to expose internal types to the test project.