feat(experimentation): expose experiment rollout in API responses

[gagantrivedi]

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Note

Stacked on #7851 (base branch feat/experiment-rollout). Review/merge that first; this PR's diff is just the read-representation on top. I'll retarget to main once #7851 merges.

Changes

Contributes to

Follow-up to #7851, which left experiment_rollout write-only. This exposes it on read so the UI can display the configured rollout.

The rollout isn't stored as a single field — on read it's reassembled from its two sources of truth:

• rollout_percentage ← the rollout segment's PERCENTAGE_SPLIT condition value
• enabled / feature_state_value / multivariate_feature_state_values ← the live segment-override feature state (resolved the same v1/v2-aware way as ExperimentFeatureSerializer does for env-level allocations)

A get_experiment_rollout(experiment) service returns this shape (mirroring the write input), and ExperimentListSerializer exposes it via a read-only experiment_rollout field — so it appears on list, retrieve, and the rollout action responses. The write-only input field stays on the base serializer (mirrors the metrics pattern); returns null when no rollout is configured.

How did you test this code?

Unit tests (all green locally) + mypy, ruff, the test linter, and docgen:

• Service: get_experiment_rollout reassembles the representation for v1 and v2 environments, and returns None when no rollout exists.
• API: GET experiment detail returns the populated experiment_rollout, and null when absent.
• Full experimentation suite (228 tests) passes — no regression from adding the field to the read representation.

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3 Skipped Deployments

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[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.59%. Comparing base (7c3d4fa) to head (a6d03fb).

Files with missing lines	Patch %	Lines
api/experimentation/services.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           feat/experiment-rollout    #7858      +/-   ##
===========================================================
- Coverage                    98.60%   98.59%   -0.01%     
===========================================================
  Files                         1473     1468       -5     
  Lines                        57669    57687      +18     
===========================================================
+ Hits                         56863    56879      +16     
- Misses                         806      808       +2     

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