feat(gax): allow non-JSON HttpContent and absolute request URLs in HttpRequestRunnable

[whowes]

GAX HTTP infrastructure currently assumes that

• request content is always JSON
• request URLs are always based on the client context associated with the service stub

This PR relaxes those assumptions to allow non-JSON content and arbitrary URLs, which will be needed for resumable upload support.

[gemini-code-assist]

Code Review

This pull request introduces support for non-JSON HTTP content and absolute URLs in HttpRequestRunnable. It adds a default getHttpContent method to HttpRequestFormatter to retrieve the request body as HttpContent, allowing custom content types like binary payloads. Additionally, HttpRequestRunnable is updated to handle absolute paths directly, bypassing endpoint normalization when the path starts with http:// or https://. Corresponding unit tests have been added to verify these changes. There are no review comments, and I have no additional feedback to provide.

[sonarqubecloud]

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

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

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

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud