fix(transport): merge default headers with custom headers (#2862)

[jerinthomascarmel]

Fix header merge logic to preserve default headers when custom headers are provided

Changes: Update header-merge logic to explicitly spread default headers and then apply opts.headers, ensuring defaults are preserved.

Closes: #2862

Testing: Run npm test to ensure the new header-merge test passes.

[cla-checker-service[bot]]

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[prodsecmachine]

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[JoshMock]

Closing in favor of #2890 which has unit tests, solves for more than one case, and does not need to wait for a CLA agreement to be signed.