Add InSampleUniformGenerator for model-free in-sample candidate selection

[ItsMrLin]

Summary:
Add InSampleUniformGenerator, a RandomGenerator subclass that randomly
selects n arms (without replacement) from existing experiment arms. This
replaces the previous in_sample mode on RandomAdapter (which bypassed the
generator entirely) with a proper generator class, following bletham's review
feedback on the original diff.

The generator overrides gen() to select from the generated_points array
that the adapter already constructs from in-design, non-failed experiment arms
(filtered, transformed, and deduplicated). This reuses existing infrastructure
without adding new interface surface.

Registered as Generators.IN_SAMPLE_UNIFORM in the adapter registry with
RandomAdapter and Cont_X_trans, matching the pattern of other random
generators (Sobol, Uniform).

The resulting user-facing matrix is clean:

	Out-of-sample	In-sample
Model-free	Generators.SOBOL	Generators.IN_SAMPLE_UNIFORM
Model-based	Generators.BOTORCH_MODULAR	Generators.BOTORCH_MODULAR + model_gen_options={"in_sample": True}

The asymmetry (enum swap for model-free, flag for model-based) reflects the
real architectural difference: model-based in/out-of-sample share a fitted
model; model-free in/out-of-sample share nothing.

Reviewed By: bletham, saitcakmak

Differential Revision: D94973263

[meta-codesync]

@ItsMrLin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D94973263.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (9f50a93) to head (8767da7).

Additional details and impacted files

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  Coverage   96.83%   96.83%           
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  Files         599      601    +2     
  Lines       64584    64662   +78     
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+ Hits        62540    62616   +76     
- Misses       2044     2046    +2     

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[meta-codesync]

This pull request has been merged in a89122d.