fix(storage): set application/json Content-Type for impersonated ADC requests

[thiyaguk09]

Summary
This PR ensures that the Content-Type header is correctly set to application/json for metadata requests when using Application Default Credentials (ADC) with service account impersonation.

Problem
When the SDK is initialized with Impersonated ADC, certain metadata-only operations (like bucket.create()) were incorrectly sending a Content-Type: text/plain header. Because the request body contains a JSON payload, the Google Cloud Storage API would fail to parse the request and return a 400 Parse Error. This issue was specifically observed as a regression in version 7.17.3.

Fix
The decorateRequest method has been updated to check for the presence of a JSON payload. If a JSON body is present and no Content-Type header has been specified, the SDK now explicitly sets the header to application/json (source). This ensures that the GCS API can correctly interpret the request regardless of the authentication method being used.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request updates util.decorateRequest to automatically set the Content-Type header to application/json when a json payload is present, without overwriting any existing content-type header. It also adds unit tests to verify this behavior for both plain header objects and Headers instances. The review feedback suggests ensuring both .set and .has methods exist on the headers object to prevent runtime errors, and adding a guard in the tests for the global Headers class to maintain compatibility with older Node.js versions.

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