feat: create endpoint to get a flag multivariate options

[Zaimwa9]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

To meet Vercel requirements, we need to provide the list of MV values in order for their Flag Explorer to list them.
It is using the SDK under the hood so it is not possible to use private keys to GET the same endpoint as the platform.
Hence why the creation of an endpoint specifically for this case

• Created a GET endpoint /api/v1/flags/{featureID}/multivariate-options/ with environment key permission level to get the list of multivariate values

Response as follows, voluntarily sending the bare minimum:

    "control_value": "test",
    "options": [
        {
            "value": 1
        },
        {
            "value": "yolotest"
        },
        {
            "value": "ortest"
        },
        {
            "value": "plop"
        }
    ]
}

Will be used as follows in vercel package (yet to be changed):

  if (flag.feature.type === 'MULTIVARIATE') {
        try {
          const mvResponse = await fetch(
            `https://api.flagsmith.com/api/v1/flags/${flag.feature.id}/multivariate-options/`,
            {
              method: 'GET',
              headers: { 'X-Environment-Key': options.environmentKey },
            },
          );
          if (mvResponse.ok) {
            const body =
              (await mvResponse.json()) as FlagsmithMultivariateApiData;
            const { control_value, options } = body;

How did you test this code?

• Added tests
• Locally

[vercel]

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Oct 8, 2025 8:35am
flagsmith-frontend-preview	Ignored	Preview		Oct 8, 2025 8:35am
flagsmith-frontend-staging	Ignored	Preview		Oct 8, 2025 8:35am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6134	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6134	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6134	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6134	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6134	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6134	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (57f0e97) to head (650f718).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6134   +/-   ##
=======================================
  Coverage   97.99%   98.00%           
=======================================
  Files        1278     1278           
  Lines       44927    45005   +78     
=======================================
+ Hits        44028    44106   +78     
  Misses        899      899           

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