Motivation and Context

This change enables host header override functionality for SigV4a signing, bringing it to parity with SigV4 signing behavior.

Background: In PR #5608, we fixed SigV4 signing to respect pre-existing Host headers rather than always overriding them. This was necessary for use cases like:

• Transparent proxies that rely on the Host header for routing
• Custom routing scenarios where the Host header differs from the endpoint

Current issue: The same fix was not applied to the SigV4a signing path, causing host header overrides to be ignored when using SigV4a.

Impact: Enhance SigV4a support such that customers expect host header override to work consistently across both signing algorithms.

Modifications

• Added host header existence check in CrtUtils.java before setting the host header in SigV4a signing flow
• This mirrors the fix previously applied to SigV4 signing in PR SigV4: Add host header only when not already provided #5608
• Ensures that if a Host header is already present in the request, it will not be overwritten during SigV4a signing

Testing

• Reproduced the issue using the provided test case showing SigV4 works but SigV4a fails
• Verified that with this change, both SigV4 and SigV4a respect pre-existing Host headers
• Regression tested to ensure default behavior (no pre-existing Host header) continues to work

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

License

• I confirm that this pull request can be released under the Apache 2 license