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 #7336

[gemini-code-assist]

Code Review

This pull request adds logic to automatically set the 'Content-Type' header to 'application/json' when a JSON body is present in the request options, supporting both plain objects and Headers instances, and includes corresponding unit tests. The feedback suggests avoiding direct mutation of the user-provided headers object to prevent side-effects, recommending a shallow copy instead.