Do not use abandoned arms as pending points; add completed trials without data as pending

[sdaulton]

Summary:
Two changes to how pending observation features are computed:

1. Do not use individually abandoned arms as pending points. Previously,
   arms that were individually abandoned within a BatchTrial (via
   mark_arm_abandoned) were explicitly added as pending observation features.
   This is removed — individually abandoned arms should not occupy the pending
   point space. Note: arms belonging to fully abandoned trials are still
   treated as pending, since the entire trial was abandoned and the model should
   avoid re-suggesting those configurations.

2. Treat completed trials without data as pending. Previously, completed
   trials were never considered pending, even if they had no data for some
   metrics. Now, get_pending_observation_features checks completed trials for
   missing metric data and marks those arms as pending for the missing metrics,
   consistent with how deployed (STAGED/RUNNING) trials are handled.

Differential Revision: D94531161

[meta-codesync]

@sdaulton has exported this pull request. If you are a Meta employee, you can view the originating Diff in D94531161.

[meta-codesync]

This pull request has been merged in 8215f32.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (c2f9aec) to head (21758f6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4955      +/-   ##
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- Coverage   96.83%   96.83%   -0.01%     
==========================================
  Files         596      596              
  Lines       63108    63103       -5     
==========================================
- Hits        61109    61103       -6     
- Misses       1999     2000       +1     

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