Include context path in S3 presigned URLs

[Jacke]

Fixes #17831

Summary

• Include the protocol context path (e.g., /storage/v1/s3 for Supabase) in presigned URLs
• Override getVirtualPath() in the anonymous RestS3Service instance used for URL signing
• This mirrors the existing behavior in RequestEntityRestStorageService for regular S3 requests

Problem

When using Cyberduck with the official Supabase Storage S3 profile (which sets Context=/storage/v1/s3), "Copy URL → Signed URL" generates a presigned URL that omits the context path prefix. The generated URL points to https://<host>/<bucket>/<key> instead of https://<host>/storage/v1/s3/<bucket>/<key>, causing Supabase to return {"error":"requested path is invalid"}.

Solution

The fix adds a getVirtualPath() override in S3PresignedUrlProvider that returns the context path from the protocol configuration. This ensures the context path is included in both:

1. The URL path of the presigned URL
2. The canonical request used for SigV4 signature calculation

Backward Compatibility

This change is backward compatible:

• For standard S3 providers (AWS, Wasabi, MinIO, etc.) where getContext() returns empty/null, getVirtualPath() returns an empty string — behavior unchanged
• Only profiles with an explicit Context setting (like Supabase) are affected

Test plan

• Added unit test testContextPath() verifying presigned URLs include the context path when configured

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[Jacke]

Thanks for a review @dkocher 👍