fix(image): only use numerical IDs for label auto-completion

[phm07]

We currently use the deprecated Get method here. Instead we can just parse an integer and use that as the ID, since you can only get or update labels on images you can modify (snapshots and backups), which only have numerical IDs and no names anyway.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 61.47%. Comparing base (311642e) to head (a138da5).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/hcapi2/image.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #865      +/-   ##
==========================================
- Coverage   61.49%   61.47%   -0.03%     
==========================================
  Files         238      238              
  Lines        8522     8525       +3     
==========================================
  Hits         5241     5241              
- Misses       2572     2575       +3     
  Partials      709      709              

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