feat(nexus): add sync file counts in the footer

[dmitryduev]

Description

What does the PR do?

🤖 Generated by Copilot at ef24ea9

This pull request adds file type counting and handling to the nexus uploader and server, improves the closing logic of the nexus handler, sender, and writer types, and fixes some minor issues in the nexus and wandb code and documentation. It also updates the nexus artifact saver to use the ArtifactFile type.

Testing

How was this PR tested?

🤖 Generated by Copilot at ef24ea9

Sing, O Muse, of the code review that shook the GitHub realm,
How the valiant developers strove to improve their project's helm,
They removed and added, fixed and renamed, with skill and care sublime,
And handled files of various types, closing them in due time.

[codecov]

Codecov Report

Merging #6371 (9774bd7) into main (2bd173a) will increase coverage by 0.75%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6371      +/-   ##
==========================================
+ Coverage   76.25%   77.00%   +0.75%     
==========================================
  Files         387      387              
  Lines       44612    44642      +30     
==========================================
+ Hits        34017    34375     +358     
+ Misses      10542    10214     -328     
  Partials       53       53              

Flag	Coverage Δ
unittest	80.88% <ø> (+0.85%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
wandb/sdk/internal/sender.py	92.80% <ø> (ø)
nexus/pkg/server/stream.go	0.00% <0.00%> (ø)
nexus/pkg/server/writer.go	0.00% <0.00%> (ø)
nexus/pkg/server/handler.go	0.00% <0.00%> (ø)
nexus/pkg/server/sender.go	16.41% <0.00%> (-0.69%)	⬇️

... and 23 files with indirect coverage changes

[kptkin]

Suggested change

	// um.fileCounts = &service.FileCounts{
	// WandbCount: 42,
	// MediaCount: 420,
	// ArtifactCount: -1,
	// OtherCount: 100500,
	// }