fix(services/azfile): avoid defaulting list size to zero

[Xuanwo]

Which issue does this PR close?

Part of #7062.

Rationale for this change

azfile lister currently treats missing Content-Length in list response as 0.
This conflates "unknown size" with "real zero-byte file", which breaks the mandatory list content-length semantics introduced in core.

What changes are included in this PR?

• Update core/services/azfile/src/lister.rs:
  • Stop using unwrap_or(0) for file content_length.
  • Set content_length only when the service response includes Content-Length.

This keeps real zero-byte files correct while allowing core fallback completion logic to handle missing sizes safely.

Validation:

• cargo fmt
• cargo check --features services-azfile,tests
• cargo test -p opendal-service-azfile

Are there any user-facing changes?

Yes.

For azfile, list entries no longer report content_length = 0 when Azure File list response omits content length.

AI Usage Statement

This PR was developed with Codex (GPT-5) assistance.