Refactor Runner base class: rename run_multiple→run_trials, add stage_trials/stop_trial/stop_arms/poll_run_metadata/apply_status_change_side_effects, deprecate run/poll_exception/stop

[lena-kashtelyan]

Summary:
Refactors the core Runner base class per the E2E Automation design doc:

Renamed/restructured methods:

• run_multiple → run_trials (kept run_multiple as deprecated wrapper)
• stop → stop_trial (kept stop as deprecated fallback)
• run is no longer abstract; run_trials default implementation calls it for backward compat

New methods added:

• stage_trials(trials) — does nothing by default, override for staging
• stop_arms(trial, arm_names, reason) — stop specific arms in a trial
• poll_run_metadata(trials) — update run metadata from actual deployment
• apply_status_change_side_effects(trials) — dedicated place for status-change side effects

Deprecated methods (to be removed after runner migration):

• run(trial) — override run_trials instead
• poll_exception(trial) — fold into poll_trial_status
• stop(trial, reason) — override stop_trial instead
• run_multiple(trials) — use run_trials instead

Differential Revision: D97842794

[meta-codesync]

@lena-kashtelyan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97842794.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.29%. Comparing base (6e527d5) to head (50a988c).

Files with missing lines	Patch %	Lines
ax/core/runner.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5094      +/-   ##
==========================================
- Coverage   96.30%   96.29%   -0.01%     
==========================================
  Files         611      611              
  Lines       67534    67566      +32     
==========================================
+ Hits        65040    65066      +26     
- Misses       2494     2500       +6     

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