bug: stream error responses are corrupted with commas due to Array.toString() in GaxiosError (AIP-193)

[jerrylin3321]

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Library Name

gaxios

A screenshot that you have tested with "Try this API".

N/A

Link to the code that reproduces this issue. A link to a public Github Repository or gist with a minimal reproduction.

https://gist.github.com/jerrylin3321/8fb4131dc2df1bce696f94fac77d90a5

A step-by-step description of how to reproduce the issue, based on the linked reproduction.

1. Initialize a new Node.js project and install gaxios v7 (e.g., npm install gaxios@7.1.3).
2. Save the reproduction script as reproduce.js.
3. Run the script using node reproduce.js.
4. Observe that the stringified response data contains a stray comma (e.g., "Invalid ,argument") injected at the chunk boundary.

A clear and concise description of what the bug is, and what you expected to happen.

See this downstream issue that it is causing in gemini-cli: google-gemini/gemini-cli#21884

A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

Regression introduced in v7.0.0 via PR #699