[WIP] Add unit tests for exception wrapping in ClaimsHelper

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Add focused unit tests to validate exception wrapping and diagnostic error message contents when PlatformNotSupportedException is thrown during JSON encoding in ClaimsHelper. The new tests should:

1. Verify that when ForceJsonPns is enabled, GetMergedClaimsAndClientCapabilities throws a MsalClientException wrapping PlatformNotSupportedException, and the error message contains key diagnostic details (ProcessArchitecture=, Is64Bit=, HWIntrinsic=).
2. Verify that the error message generator handles a null environment variable gracefully, including '(not set)' in the message as appropriate.

Tests should follow the Arrange/Act/Assert style and match the conventions used in existing ClaimsTest.cs tests.

*This pull request was created as a result of the following prompt from Copilot chat.* > Add focused unit tests to validate exception wrapping and diagnostic error message contents when PlatformNotSupportedException is thrown during JSON encoding in ClaimsHelper. The new tests should: > > 1. Verify that when ForceJsonPns is enabled, GetMergedClaimsAndClientCapabilities throws a MsalClientException wrapping PlatformNotSupportedException, and the error message contains key diagnostic details (ProcessArchitecture=, Is64Bit=, HWIntrinsic=). > 2. Verify that the error message generator handles a null environment variable gracefully, including '(not set)' in the message as appropriate. > > Tests should follow the Arrange/Act/Assert style and match the conventions used in existing ClaimsTest.cs tests.

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