feat(gax): add getFirstHeader to HttpHeadersUtils

[whowes]

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[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, supporting both single values and iterables. It also adds comprehensive unit tests in HttpHeadersUtilsTest to verify this new functionality. There are no review comments, and I have no additional 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

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

[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