fix: Webhook payloads do not include multivariate values

[khvn26]

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Changes

Contributes to #6190.

In this PR, we add multivariate_feature_state_values to environment webhook payloads for FLAG_UPDATED events. The payload now includes each multivariate option's ID, value, and percentage allocation in both new_state and previous_state:

"multivariate_feature_state_values": [
  {
    "id": 1,
    "multivariate_feature_option": {
      "id": 10,
      "value": "variant_a"
    },
    "percentage_allocation": 50
  }
]

The documentation and sample webhook value are updated accordingly.

How did you test this code?

Added an integration test reproducing the issue, and will test in staging.

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview, Comment	Feb 5, 2026 9:48pm

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ignored	Preview	Feb 5, 2026 9:48pm
flagsmith-frontend-staging	Ignored	Preview	Feb 5, 2026 9:48pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6666	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-6666	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6666	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6666	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6666	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6666	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (f644ab2) to head (aa79ef9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6666   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files        1298     1298           
  Lines       47108    47169   +61     
=======================================
+ Hits        46262    46323   +61     
  Misses        846      846           

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