bugfix: Fix clientAssertionSigningKey type mismatch

[tusharpandey13]

This pull request resolves a TypeScript error by aligning the clientAssertionSigningKey type with the jose library. This ensures the key is correctly processed for client assertion signing.

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

🔍 RCA

The clientAssertionSigningKey property was typed as a generic CryptoKey, but it was being assigned a jose.CryptoKey. This created a type conflict because jose.CryptoKey is not directly assignable to the Web Crypto CryptoKey type.

📋 Changes

The changes align the type definitions and ensure the key is correctly cast when used.

• Changed src/server/auth-client.test.ts: Updated the test helper's privateKey parameter to use jose.CryptoKey.
• Changed src/server/auth-client.ts: Updated clientAssertionSigningKey to use jose.CryptoKey and cast it to CryptoKey when calling oauth.PrivateKeyJwt.

🎯 Testing

Automated:
The existing test suite covers the client assertion signing logic and continues to pass with these type corrections.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.41%. Comparing base (533c4b7) to head (289bc83).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2243   +/-   ##
=======================================
  Coverage   84.40%   84.41%           
=======================================
  Files          26       26           
  Lines        2379     2380    +1     
  Branches      442      442           
=======================================
+ Hits         2008     2009    +1     
  Misses        365      365           
  Partials        6        6           

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