fix(services/cos): preserve list metadata etag and last_modified

[suyanhanx]

Which issue does this PR close?

Closes #7306.

Rationale for this change

COS list responses include ETag and LastModified, but the normal list path only deserialized key and size, so list metadata lost those fields even though stat() could return them.

What changes are included in this PR?

• deserialize LastModified and ETag in COS ListObjectsOutputContent
• populate last_modified, etag, and content_md5 in the normal COS lister path
• extend the COS XML parsing unit test to cover both fields

Are there any user-facing changes?

Users of COS list operations can now read etag and last_modified metadata from list results consistently across bindings.

AI Usage Statement

Used Codex (GPT-5) to investigate the issue, implement the fix, run tests, and validate the behavior with a Python reproduction script.