[Identity] Updated proper claims encoding

[minhanh-phan]

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• Added impacted package name to the issue description
• Does this PR needs any fixes in the SDK Generator?** (If so, create an Issue in the Autorest/typescript repository and link it here)
• Added a changelog (if necessary)

[Copilot]

Pull Request Overview

This PR fixes an issue where AzureCliCredential, AzurePowerShellCredential, and AzureDeveloperCliCredential were not properly encoding claims challenges as base64 strings before including them in command line arguments.

• Updated all three credential implementations to encode claims values using btoa() before passing them to CLI commands
• Updated corresponding test files to expect base64-encoded claims in command assertions
• Added changelog entry documenting the bug fix

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

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File	Description
sdk/identity/identity/src/credentials/azureCliCredential.ts	Added base64 encoding of claims challenges before constructing CLI commands
sdk/identity/identity/src/credentials/azurePowerShellCredential.ts	Added base64 encoding of claims challenges before constructing PowerShell commands
sdk/identity/identity/src/credentials/azureDeveloperCliCredential.ts	Added base64 encoding of claims challenges before constructing Azure Developer CLI commands
sdk/identity/identity/test/internal/node/azureCliCredential.spec.ts	Updated test assertions to expect base64-encoded claims in CLI commands
sdk/identity/identity/test/internal/node/azurePowerShellCredential.spec.ts	Updated test assertions to expect base64-encoded claims in PowerShell commands
sdk/identity/identity/test/internal/node/azureDeveloperCliCredential.spec.ts	Updated test assertions to expect base64-encoded claims in Azure Developer CLI commands
sdk/identity/identity/CHANGELOG.md	Added changelog entry documenting the claims encoding bug fix