LILO hash computation and stamping for data freshness

[ItsMrLin]

Summary:
Add hash-based data freshness tracking for LILO (Language-in-the-Loop)
pairwise preference labels.

When LILOPairwiseMetric produces labels, it now stamps a SHA-256 hash of the
experiment's LILO inputs (metric data for input_metric_names + LLM messages)
onto the trial's _properties. If any of these inputs change (new data arrives,
data is updated, or the user modifies LLM messages), the hash changes,
indicating that existing LILO labels are stale.

Changes:

• Add LILO_INPUT_HASH key to Keys enum in constants.py
• Create ax/utils/common/hash_utils.py with compute_lilo_input_hash
  (standalone hash function) and get_current_lilo_hash (convenience helper
  that looks up the pairwise DerivedMetric on an experiment, extracts
  input_metric_names, and computes the hash — returns None if no pairwise
  metric is registered)
• Stamp hash in LILOPairwiseMetric._compute_derived_values after producing labels
• Add tests for hash determinism, sensitivity to data/message changes, stamping,
  and get_current_lilo_hash helper

Reviewed By: saitcakmak

Differential Revision: D95284287

[meta-codesync]

@ItsMrLin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95284287.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 3.70370% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.80%. Comparing base (3e8e4e7) to head (942b0f9).

Files with missing lines	Patch %	Lines
ax/utils/common/hash_utils.py	0.00%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4992      +/-   ##
==========================================
- Coverage   96.84%   96.80%   -0.04%     
==========================================
  Files         601      602       +1     
  Lines       64732    64759      +27     
==========================================
+ Hits        62687    62689       +2     
- Misses       2045     2070      +25     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.