feat(api): add sealed exception hierarchy for api plugin

[jvh-aws]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:

Adds sealed exception hierarchy for API plugin enabling exhaustive Kotlin when expressions while maintaining backward compatibility with existing catch blocks.

New types marked @ExperimentalAmplifyApi, opt in with @OptIn(ExperimentalAmplifyApi::class) to use. ApiAuthException made non-final (non-breaking) to allow subclassing; NonRetryableException is otherwise unchanged to avoid a breaking change.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(storage): message, feat(auth): message, chore(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.93%. Comparing base (1aca347) to head (b44c44e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3279   +/-   ##
=======================================
  Coverage   55.92%   55.93%           
=======================================
  Files        1083     1084    +1     
  Lines       31932    31952   +20     
  Branches     4760     4760           
=======================================
+ Hits        17859    17873   +14     
- Misses      12219    12223    +4     
- Partials     1854     1856    +2     

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