feat(gax): add getFirstHeader to HttpHeadersUtils

[whowes]

The GAX HTTP/JSON stack stores response headers in a Map<String, Object> where values may be Strings, numbers, Iterables, etc., and header names may vary in casing depending on the HTTP version. This method provides case-insensitive lookup and safely extracts the first string value in a typesafe way.

This will be used in resumable upload support to retrieve protocol header values (e.g. X-Goog-Upload-URL, Location, and X-Goog-Upload-Status).

[gemini-code-assist]

Code Review

This pull request introduces a new utility method getFirstHeader in HttpHeadersUtils to retrieve the first string value of a header by case-insensitive name from a headers map, along with a helper method extractFirstString to handle different header value types (such as iterables and non-string objects). Comprehensive unit tests have also been added to cover various scenarios, including case-insensitivity, iterable values, null values, and non-existent headers. There are no review comments, and I have no feedback to provide.

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

Catch issues before they fail your Quality Gate with our IDE extension SonarQube for IDE