feat(experimentation): default to control when rollout segment is created

[gagantrivedi]

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Changes

Contributes to experiment rollout work.

Warning

WIP / draft.

When a feature is first added to an experiment's rollout — i.e. the moment the rollout system segment is created — we now zero every variant's allocation on the feature's environment-default feature state. Control is the implicit unallocated remainder, so this gives control 100% of out-of-rollout traffic while the experiment runs.

• The rollout segment override is unchanged: it keeps the experiment's own split (spec.multivariate_values).
• This only runs when the rollout segment is first created. Tuning the rollout of a running/paused experiment leaves the default allocations alone, so a later manual edit to the default survives.
• Reuses the version-aware update_flag path (v1/v2) rather than mutating allocations in place.

How did you test this code?

Added unit tests (make test):

• Service (test_services.py):
  • test_apply_experiment_rollout__first_rollout__zeroes_default_allocations — default allocations zeroed, override split preserved.
  • test_apply_experiment_rollout__existing_segment__leaves_default_allocations — a later manual edit to the default survives a rollout update.
• View (test_experiment_views.py):
  • test_post__with_experiment_rollout__zeroes_default_allocations — end-to-end create-with-rollout zeroes the default and preserves the override split (both user and master_api_key).

mypy, ruff, and flagsmith-lint-tests all clean.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Jun 30, 2026 5:17am
flagsmith-frontend-preview	Ignored	Preview	Jun 30, 2026 5:17am
flagsmith-frontend-staging	Ignored	Preview	Jun 30, 2026 5:17am

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.56%. Comparing base (1d1cc9c) to head (8275f6d).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
api/experimentation/services.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7905      +/-   ##
==========================================
- Coverage   98.62%   98.56%   -0.07%     
==========================================
  Files        1488     1488              
  Lines       58543    58558      +15     
==========================================
- Hits        57737    57716      -21     
- Misses        806      842      +36     

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