feat(fs): support Deno.copyFile for S3-backed file systems

[da1suk8]

Summary

Adds Deno.copyFile and Deno.copyFileSync support for S3-backed file systems.

• Copies objects with S3's CopyObject API without downloading data locally
• Maps missing source objects/buckets to NotFound
• URL-encodes copy-source object keys correctly
• Adds unit and integration test coverage for async and sync copies

Limitation

This implementation relies on S3 CopyObject, which supports source objects up to 5 GiB.
Copying larger objects will require multipart copy support in a follow-up change.

Validation

• cargo fmt --all -- --check
• cargo metadata --no-deps --format-version=1 --locked --offline
• cargo test -p fs --lib copy_