feat(sweeps): add Sweep.expected_run_count to public Api

[gtarpenning]

https://wandb.atlassian.net/browse/WB-11414

Description

Adds expected run count -- now with a legacy fallback query

Testing

added testing in test_public_api.py

Checklist

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

[codecov]

Codecov Report

Merging #4434 (35e2c68) into main (61bdf1a) will decrease coverage by 0.02%.
The diff coverage is 69.23%.

❗ Current head 35e2c68 differs from pull request most recent head ad3a199. Consider uploading reports for the commit ad3a199 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4434      +/-   ##
==========================================
- Coverage   83.05%   83.03%   -0.03%     
==========================================
  Files         259      258       -1     
  Lines       32906    32876      -30     
==========================================
- Hits        27330    27298      -32     
- Misses       5576     5578       +2     

Flag	Coverage Δ
functest	56.87% <23.07%> (+0.05%)	⬆️
unittest	73.14% <69.23%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
wandb/apis/public.py	81.80% <69.23%> (-0.36%)	⬇️
wandb/sklearn/plot/classifier.py	88.15% <0.00%> (-8.91%)	⬇️
wandb/sdk/launch/builder/loader.py	80.00% <0.00%> (-3.34%)	⬇️
wandb/sdk/service/service.py	89.15% <0.00%> (-0.13%)	⬇️
wandb/sdk/wandb_run.py	90.91% <0.00%> (-0.12%)	⬇️
wandb/sdk/launch/utils.py	92.48% <0.00%> (-0.11%)	⬇️
wandb/sdk/wandb_artifacts.py	83.71% <0.00%> (-0.11%)	⬇️
wandb/sdk/internal/settings_static.py	93.75% <0.00%> (-0.10%)	⬇️
wandb/sdk/internal/system/assets/ipu.py	95.74% <0.00%> (-0.05%)	⬇️
... and 10 more