fix: Webhook payloads do not include multivariate values

[khvn26]

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• I have filled in the "Changes" section below.
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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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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 6, 2026 3:28pm
flagsmith-frontend-preview	Ignored	Preview	Feb 6, 2026 3:28pm
flagsmith-frontend-staging	Ignored	Preview	Feb 6, 2026 3:28pm

[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 98.22%. Comparing base (f644ab2) to head (de364e4).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6666      +/-   ##
==========================================
+ Coverage   98.20%   98.22%   +0.01%     
==========================================
  Files        1298     1313      +15     
  Lines       47108    47481     +373     
==========================================
+ Hits        46262    46636     +374     
+ Misses        846      845       -1     

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[Zaimwa9]

1 NIT and a comment, otherwise clean