feat: remove ForceAttemptOAuth2 add SetLegacyMode to auth client

[TerryHowe]

Remove ForceAttemptOAuth2 and add SetLegacyMode to the auth client.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.59%. Comparing base (4427369) to head (55bef2f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   82.50%   82.59%   +0.09%     
==========================================
  Files          64       64              
  Lines        4779     4781       +2     
==========================================
+ Hits         3943     3949       +6     
+ Misses        512      509       -3     
+ Partials      324      323       -1     

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[Copilot]

Pull request overview

This PR refactors the authentication client API by replacing the public ForceAttemptOAuth2 field with a private legacyMode field and introducing a new SetLegacyMode method. The default authentication behavior changes from using the legacy distribution spec to using OAuth2 with password grant.

Key changes:

• Replaced public ForceAttemptOAuth2 boolean field with private legacyMode field and SetLegacyMode(bool) method
• Inverted default behavior: OAuth2 is now the default instead of legacy distribution spec
• Updated all tests to use the new API pattern

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
registry/remote/auth/client.go	Replaced ForceAttemptOAuth2 field with legacyMode field, added SetLegacyMode method, and inverted the authentication logic to make OAuth2 the default
registry/remote/auth/client_test.go	Removed ForceAttemptOAuth2: true from OAuth2 tests (now use default), added SetLegacyMode(true) calls to legacy distribution spec tests
registry/remote/auth/example_test.go	Removed ForceAttemptOAuth2: true from example to use the new OAuth2 default behavior

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[shizhMSFT]

Since we are now in v3 development, should we just remove ForceAttemptOAuth2?

[TerryHowe]

Since we are now in v3 development, should we just remove ForceAttemptOAuth2?

This pull request does remove ForceAttemptOAuth2 I don't think that will have a big impact on users.