fix: stream downloader artifacts

[Priveetee]

Summary

• Stream internal Garage artifact redirects instead of buffering them in TypeType-Server.
• Preserve Range responses for downloader artifacts.
• Remove the full-body artifact fetch helper and cover the internal redirect path with a test.

Verification

• JAVA_HOME=/usr/lib/jvm/java-21-openjdk ./gradlew test
• JAVA_HOME=/usr/lib/jvm/java-21-openjdk ./gradlew shadowJar
• CI, Coverage, and Docker succeeded on dev push.