fix: capture async stack only for rejections with native error object…

[MitchLewis930]

PR_025

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Note

Low Risk
Tightly scoped change to error stack formatting and a related unit test; main risk is subtle differences in stack strings across runtimes.

Overview
Improves error stack preservation in Axios.request() by only capturing and appending the async call-site stack when the rejection is a native Error, and by handling cases where err.stack is missing before merging.

Updates the HTTP adapter unit test for stack preservation to use async/await server startup (startHTTPServer) and return the assert.rejects promise, keeping the single-occurrence stack assertion intact.

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