Change get_trace to return dict[int, float] and filter by MetricAvailability

[saitcakmak]

Summary:
Goal: Eliminate assumptions around COMPLETED trials always having all metrics available. This is part of the code that would've raised errors if it encountered missing metrics in COMPLETED trials. Changing the return type here seemed like a better move than using carry-forward logic, and fits better with the logic that skips status quo as well (which may lead to confusing indexing when skipping status quo).

How does this new return type fit with BatchTrial: get_trace takes the best arm out of each batch, so it returns (and did so before this change) one result per trial. Nothing changed.

Changes (claude summary):
get_trace previously returned list[float] with positional indexing,
which forced callers to fabricate sequential trial indices. This change:

1. Changes the return type to dict[int, float] mapping trial_index to
   performance value, so callers get real trial indices.

2. Adds MetricAvailability filtering to exclude trials with incomplete
   metric data before pivoting. This prevents the ValueError("Some metrics are not present for all trials and arms") that
   _pivot_data_with_feasibility raises when a completed trial is
   missing any metric (e.g., partial fetches, fetch failures, metrics
   added mid-experiment).

3. Removes the carry-forward expansion loop for abandoned/failed trials.
   These trials are now simply excluded from the returned dict.

Callers updated:

• UtilityProgressionAnalysis now shows real trial indices on x-axis
• pareto_frontier.hypervolume_trace_plot uses real trial indices
• benchmark.py extracts values from dict

Differential Revision: D99448053

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99448053.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.40%. Comparing base (227243c) to head (0590cdd).

Files with missing lines	Patch %	Lines
ax/service/utils/best_point.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5140   +/-   ##
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  Coverage   96.40%   96.40%           
=======================================
  Files         613      613           
  Lines       68191    68194    +3     
=======================================
+ Hits        65740    65743    +3     
  Misses       2451     2451           

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