[Identity] Improve WorkloadIdentityCredential/DAC diagnosability

[pvaneck]

The goal here is to have WorkloadIdentityCredential be reported in the final error message as an attempted credential.

A substitute credential for credentials that fail to construct has been added and used in the WorkloadIdentityCredential case. This will allow the reason WorkloadIdentityCredential can't be used to be propagated to the user.

An error message like the following will now be printed with the reason WorkloadIdentityCredential was not attempted.

Attempted credentials:
        EnvironmentCredential: EnvironmentCredential authentication unavailable. Environment variables are not fully configured.
Visit https://aka.ms/azsdk/python/identity/environmentcredential/troubleshoot to troubleshoot this issue.
        WorkloadIdentityCredential: WorkloadIdentityCredential authentication unavailable. The workload options are not fully configured. See the troubleshooting guide for more information: https://aka.ms/azsdk/python/identity/workloadidentitycredential. Missing required arguments: 'tenant_id', 'client_id', 'token_file_path'.
        ManagedIdentityCredential: ManagedIdentityCredential authentication unavailable. The requested identity has not been assigned to this resource. Error: Unexpected response "{'error': 'invalid_request', 'error_description': 'Identity not found'}"
        SharedTokenCacheCredential: SharedTokenCacheCredential authentication unavailable. No accounts were found in the cache.
...
To mitigate this issue, please refer to the troubleshooting guidelines here at https://aka.ms/azsdk/python/identity/defaultazurecredential/troubleshoot.

Closes: #42075

[Copilot]

Pull Request Overview

This PR improves error diagnostics for the WorkloadIdentityCredential within the DefaultAzureCredential chain by introducing a substitute credential mechanism that captures initialization errors and reports them in the final error message when credential acquisition fails.

• Introduces FailedDACCredential and AsyncFailedDACCredential classes to capture and report initialization errors
• Updates error message generation logic to use custom credential names from failed credential instances
• Modifies WorkloadIdentityCredential error handling to provide more informative error messages with troubleshooting guidance

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
azure/identity/_credentials/default.py	Adds FailedDACCredential class and error handling for WorkloadIdentityCredential initialization
azure/identity/aio/_credentials/default.py	Adds async version AsyncFailedDACCredential and corresponding error handling
azure/identity/_credentials/workload_identity.py	Updates error messages to use standardized format with troubleshooting guidance
azure/identity/aio/_credentials/workload_identity.py	Updates async version with same standardized error messages
azure/identity/_credentials/chained.py	Modifies error message generation to support custom credential names
tests/test_default.py	Adds comprehensive tests for the new failed credential functionality
tests/test_default_async.py	Adds async tests for the new failed credential functionality
CHANGELOG.md	Documents the improvement in diagnostics