impl(gax): refactor SendError to use BoxError

[suzmue]

Refactor SendError in google-cloud-gax to capture BoxError directly in the Serialization variant instead of wrapping crate::error::Error.

Remove temporary stream_closed and ser constructors now that construction is handled directly via SendError::StreamClosed and SendError::Serialization. These were intended to make refactoring the error type easier, but I have decided to keep an enum instead of doing an opaque struct with an internal SendErrorKind.

[gemini-code-assist]

Code Review

This pull request refactors SendError in the gax crate by removing internal helper methods, simplifying the enum variants, and implementing From<SendError> for crate::error::Error. The review feedback recommends preserving the underlying serialization error details in the Display representation of SendError::Serialization to ensure errors remain actionable and easy to debug, in line with the repository's style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.39%. Comparing base (5e2c1e0) to head (9529563).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6475      +/-   ##
==========================================
- Coverage   96.39%   96.39%   -0.01%     
==========================================
  Files         301      301              
  Lines       84855    84844      -11     
==========================================
- Hits        81800    81789      -11     
  Misses       3055     3055              

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[coryan]

Is this still needed if we don't change the examples?

Add SendError::into_error to explicitly convert a SendError into a
gax::error::Error without implementing the From trait.

Hmmm.... I am not sure "without implementing the From trait" is a win....

[suzmue]

is still needed if we don't change the examples?

Add SendError::into_error to explicitly convert a SendError into a
gax::error::Error without implementing the From trait.

Hmmm.... I am not sure "without implementing the From trait" is a win....

When I tried to implement From I got all sorts of errors across our crates that it could no longer determine which type something was. It is totally possible this isn't a big deal and I just didn't look into it closely enough and there was a simple fix (I did not look that hard for one). I will remove the conversion since its unneeded and can look into whether we can implement From without breaking anything, because that is what I wanted to do at first.