fix(s3): set content length for copy object

[ByteBaker]

Which issue does this PR close?

Closes #7821.

Rationale for this change

Some S3-compatible services require Content-Length to be present on CopyObject requests, even when the request body is empty. OpenDAL already sends an empty body for S3 copy, so this makes the request explicit by setting Content-Length: 0.

What changes are included in this PR?

• Set Content-Length: 0 when building S3 CopyObject requests.
• Split CopyObject request construction into s3_copy_object_request, matching existing request-builder patterns.
• Add a unit test that verifies the CopyObject request method, headers, and empty body.

Are there any user-facing changes?

No API changes. This improves compatibility with S3-compatible services.

AI Usage Statement

I used OpenAI Codex to help implement and test this change.

[dentiny]

Just for my own curiosity, is there any doc mentioning netapp S3 gateway requires content-length header? I didn't find it after some seach

[Xuanwo]

Other changes LGTM

[Xuanwo]

Why change this?

[ByteBaker]

I needed to test the constructor in a UT, but the construction was not separate, it was happening inside impl Builder, which returned Service, not S3Backend.

Doing this allowed me to test on S3Backend itself w/o duplicating much code.

[ByteBaker]

Just for my own curiosity, is there any doc mentioning netapp S3 gateway requires content-length header? I didn't find it after some seach

@dentiny even I couldn't find one that explicitly makes it mandatory. However, I did find their error listing which has one MissingContentLength that makes me think they care about this. Other than this, not really.