fix: add loading state for MultiSelect in Create Segment modal

[talissoncosta]

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Changes

Closes #6544

Adds a loading state check for the MultiSelect component in RuleConditionValueInput to prevent crashes when opening the Create Segment modal with environment-based operators.

Root Cause: When showMultiEnvironmentSelect is true but projectId is undefined or environments haven't loaded yet, react-select crashes internally with:

• Chrome: TypeError: Cannot read properties of null (reading 'reduce')
• Safari: null is not an object (evaluating 'e.options.reduce')

Fix:

• Added isLoading check for when projectId is undefined or data?.results hasn't loaded
• When loading: disable the MultiSelect, show "Loading environments..." placeholder, and use empty selectedValues
• When loaded: normal behaviour with environments available

Sentry Issues:

• FLAGSMITH-FRONTEND-4G8 - 3 events, 1 user
• FLAGSMITH-FRONTEND-4G9 - 2 events, 1 user
• FLAGSMITH-FRONTEND-4DP - 1 event, 1 user
• FLAGSMITH-FRONTEND-4DQ - 1 event, 1 user

How to reproduce

1. Navigate to a project's Segments page (/project/{id}/segments)
2. Click "Create Segment" button
3. Add a rule condition
4. Select the property environment
5. Select an operator that triggers multi-environment selection (e.g., IN, NOT_IN)
6. Before fix: Modal crashes with TypeError
7. After fix: MultiSelect shows "Loading environments..." until data is ready, then displays environment options

Alternative reproduction (simulating the race condition):

1. Open Chrome DevTools → Network tab
2. Set throttling to "Slow 3G"
3. Follow steps 1-5 above
4. The loading state should be visible briefly before environments load

How did you test this code?

• ESLint passed via pre-commit hook
• Manual testing following the reproduction steps above
• Verified the MultiSelect displays correctly once environments are loaded
• Verified the disabled state prevents interaction while loading

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Project	Deployment	Review	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview, Comment	Jan 20, 2026 11:43pm
flagsmith-frontend-staging	Ready	Preview, Comment	Jan 20, 2026 11:43pm

1 Skipped Deployment

Project	Deployment	Review	Updated (UTC)
docs	Ignored		Jan 20, 2026 11:43pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6565	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6565	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6565	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6565	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6565	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6565	Finished ✅	Results ✅