feat(nexus): add support for policy=end in wandb.save

[kptkin]

Fixes

• Fixes WB-NNNNN
• Fixes #NNNN

Description

What does the PR do?

🤖 Generated by Copilot at 094b530

Add caching for files with END or LIVE policy in handler.go. This improves the efficiency and consistency of file handling on the server.

Testing

How was this PR tested?

Checklist

• Include reference to internal ticket "Fixes WB-NNNNN" and/or GitHub issue "Fixes #NNNN" (if applicable)
• Ensure PR title compliance with the conventional commits standards

🤖 Generated by Copilot at f35fe91

Sing, O Muse, of the cunning Handler, who tracked the files
That he sent to the cloud with swift and varied policies,
And when his task was done, he sent a final message clear
To tell the mighty backend all the deeds that he had wrought.

[codecov]

Codecov Report

Merging #6267 (fe1a6d8) into main (0f45a1e) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6267      +/-   ##
==========================================
- Coverage   77.17%   76.99%   -0.18%     
==========================================
  Files         384      385       +1     
  Lines       44301    44352      +51     
==========================================
- Hits        34189    34150      -39     
- Misses      10059    10149      +90     
  Partials       53       53              

Flag	Coverage Δ
unittest	80.74% <ø> (-0.10%)	⬇️

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

Files Changed	Coverage Δ
nexus/pkg/server/files.go	0.00% <0.00%> (ø)
nexus/pkg/server/handler.go	0.00% <0.00%> (ø)

... and 11 files with indirect coverage changes