Fix InSampleUniformGenerator selecting arms from CANDIDATE trials

[ItsMrLin]

Summary:
When LILO labeling generates pairwise comparisons, InSampleUniformGenerator selects 2 arms uniformly at random from generated_points built by RandomAdapter. Previously, this pool included arms from CANDIDATE trials (which have no observed data) via two sources:

1. arms_by_signature_for_deduplication (all non-FAILED arms)
2. pending_observations (CANDIDATE/STAGED/RUNNING arms appended for dedup)

When a CANDIDATE trial arm was selected, LILOPairwiseMetric could not find source metric data for it, causing fetch_data to raise an ExceptionGroup.

Fix: In RandomAdapter._gen(), when the generator is InSampleUniformGenerator:

• Filter arms_to_deduplicate to only arms from expecting_data trials (COMPLETED, EARLY_STOPPED, RUNNING)
• Skip appending pending_observations (which re-adds CANDIDATE arms)

This is correct for all current and foreseeable use cases of InSampleUniformGenerator. The generator is used in two scenarios: LILO labeling (active) and potentially bandit candidate generation (not currently using it). In both cases, we need arms with observed data -- selecting an arm from a CANDIDATE or STAGED trial that has never been evaluated is never meaningful. Excluding pending observations is similarly safe: pending points exist to prevent regular generators (Sobol, BO) from re-suggesting in-flight arms, but for in-sample selection the entire point is to pick from already-observed arms.

The change is gated behind an isinstance check -- no effect on other generators (Sobol, Uniform, etc.).

Reviewed By: saitcakmak

Differential Revision: D98627073

[meta-codesync]

@ItsMrLin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D98627073.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.41%. Comparing base (3bcf8c4) to head (cb2b1fb).

Additional details and impacted files

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##             main    #5109   +/-   ##
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  Coverage   96.41%   96.41%           
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  Files         613      613           
  Lines       68106    68131   +25     
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+ Hits        65663    65688   +25     
  Misses       2443     2443           

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[meta-codesync]

This pull request has been merged in 218bf55.