Avoid the copy operation during Maven package uploads

What does this MR do and why?

Avoids the object-storage copy operation during Maven package uploads by writing files directly to their final storage path, mirroring the pattern already used for Generic Packages (!147454 (merged)) and the Terraform Module Registry. Gated behind a default-off flag for rollout and monitoring, per the epic's plan.

Also sets an explicit Content-Disposition header on Maven package downloads, for parity with the fix for the equivalent Generic Packages issue. Maven's download path already avoids that bug class for an unrelated reason (it forces a Workhorse-proxied response for custom checksum headers rather than a redirect to a presigned object-storage URL), but setting the header explicitly is defense-in-depth.

References

• #439809
• Epic: gitlab-org#12565
• Prior art: !147454 (merged) (Generic Packages)
• Related historical bug fixed for Generic Packages: #465102 (closed), #492354 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure Object Storage is enabled in your GDK.
2. Open the trace tool in the MinIO web interface (or run mc admin trace -a gdklocal) and start tracing.
3. Push a Maven package file, for example via mvn deploy:deploy-file or:

   curl --header "PRIVATE-TOKEN: <PAT>" --upload-file ./dummy.jar \
     "http://gdk.test:3000/api/v4/projects/<project_id>/packages/maven/com/example/my-app/1.0/my-app-1.0.jar"

4. In the trace, you should see PutObject, CopyObject and DeleteObject.
5. In the rails console, enable the flag:

   Feature.enable(:skip_copy_operation_in_maven_packages_upload)

6. Push another Maven package file while watching the trace — now only PutObject runs.
7. Download the package back — it should work normally and include a Content-Disposition: attachment; filename="..." header.

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