fix(gaxios): prevent comma corruption when draining stream error responses

[pearigee]

With this fix, we are hoping to address google-gemini/gemini-cli#21884 and #8768.

Credit to @jerrylin3321 for originally creating this branch. We are creating a PR to add a few additional changes on top of it.

[gemini-code-assist]

Code Review

This pull request addresses an issue where stream error responses split across multiple chunks were corrupted with commas during string conversion. It updates the response parsing logic to join string chunks directly or concatenate them as Buffers if they contain non-string chunks. A test case was added to verify this behavior with string chunks. The reviewer suggested adding an additional test case using Buffer chunks to ensure full coverage of the fallback concatenation logic.