feat(artifacts): add unit tests for azure ref arts downloads

[amusipatla-wandb]

Description

• Fixes WB-21487

This PR adds unit tests and mocks for the azure file transfer download implementation. This specifically tests the following cases:

1. erroring with invalid reference (not s3 ref)
2. erroring with an invalid s3 reference (valid format but no object)
3. downloading an unversioned obj
4. downloading the given version when it exists
5. erroring when no version has a matching checksum.
6. downloading all the items under the reference when uploaded with checksum=False (and so digest = reference)

in particular due to limitations with mocking + overriding clients in tests, this does not include a test for finding and downloading the correct version when versionId not given (and latest obj doesn't have a matching etag), which was included in the s3 unit tests.

• I updated CHANGELOG.md, or it's not applicable

Testing

How was this PR tested?

[amusipatla-wandb]

added these wrappers because of the self-referential return value of WithVersionId. open to ideas on other ways to deal with that

[codecov]

Codecov Report

Attention: Patch coverage is 49.01961% with 26 lines in your changes missing coverage. Please review.

Project coverage is 68.91%. Comparing base (f03a814) to head (68d93a1).

Files with missing lines	Patch %	Lines
core/internal/filetransfer/file_transfer_azure.go	48.00%	24 Missing and 2 partials ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           amusipatla/azure-file-transfer    #8801      +/-   ##
==================================================================
+ Coverage                           68.72%   68.91%   +0.19%     
==================================================================
  Files                                 549      549              
  Lines                               58131    58165      +34     
==================================================================
+ Hits                                39949    40086     +137     
+ Misses                              17541    17412     -129     
- Partials                              641      667      +26     

Flag	Coverage Δ
func	20.03% <ø> (ø)
system	55.81% <11.76%> (-0.02%)	⬇️
unit	50.72% <41.17%> (+0.23%)	⬆️

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

Files with missing lines	Coverage Δ
core/internal/filetransfer/file_transfers.go	100.00% <100.00%> (ø)
core/internal/filetransfer/file_transfer_azure.go	52.96% <48.00%> (+47.77%)	⬆️

... and 4 files with indirect coverage changes