Fix two-sided outcome-constraint metrics showing a single direction

[tg9963]

Summary:
A metric constrained on both sides (a two-sided band with both a >= and a
<= outcome constraint, e.g. m >= 0.9 and m <= 1.1) was rendered on the Ax
platform with a single "increase" direction, so the upper bound appeared to be
lost. The constraint op and bound were always stored correctly; the bug was
in the metric's derived lower_is_better.

When building the experiment, Ax derived each metric's lower_is_better from
only the FIRST outcome constraint referencing it (if metric_name not in map).
For a two-sided band the first constraint is >= (GEQ -> lower_is_better=False
-> "Increase"), and the second bound was ignored. A metric bounded on both sides
has no single preferred direction.

• Extract a shared InstantiationBase._build_lower_is_better_map helper. A
  metric with conflicting constraint ops (both GEQ and LEQ) now maps to
  lower_is_better=None instead of the arbitrary first constraint's direction.
• Use the helper in both call sites that previously duplicated the buggy block:
  make_experiment (create path) and AxClient.set_optimization_config
  (update path). Drop the now-unused ComparisonOp import in ax_client.

Objective directions and single-sided constraints are unchanged. Objective and
constraint metric names are always disjoint (an objective metric cannot be
constrained), so the two groups never conflict.

Differential Revision: D113506683

[meta-codesync]

@tg9963 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D113506683.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (f5c0218) to head (fda69e1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5255      +/-   ##
==========================================
- Coverage   96.58%   96.57%   -0.01%     
==========================================
  Files         621      621              
  Lines       70663    70682      +19     
==========================================
+ Hits        68248    68262      +14     
- Misses       2415     2420       +5     

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