Enhance Remote Feature Flag Overrides for Controller-Level Integration

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### What is this about?

Extend the current remote feature flag override functionality to directly inject overrides into the `RemoteFeatureFlagController`, allowing for comprehensive testing of controller-level integrations without requiring network request mocking.

**Override Precedence (Highest to Lowest)**

1. Controller-level overrides - Passed directly to the controller constructor
2. App-level overrides - From `.manifest-overrides.json` at app startup
3. Network-fetched values - The actual remote feature flag values from the LaunchDarkly server

### Scenario

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### Design

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### Technical Details

Add an optional constructor parameter to RemoteFeatureFlagController that allows direct injection of feature flag overrides:
```
const controller = new RemoteFeatureFlagController({
// Other existing options
featureFlagOverrides: {
// Object containing flag overrides
"feature1": true,
"feature2": false
}
});
```

**Implementation Notes**

- The controller should prioritize injected overrides over network-fetched values
- This approach keeps the controller agnostic of manifest files or specific file formats
- Existing override functionality should continue to work as before

### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

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Locate the RemoteFeatureFlagController entry point and trace the existing app-level manifest override handling and network-fetched values. Review the controller's existing tests before changing its constructor. Done means injected overrides take precedence, existing override behavior remains intact, and controller-level integration coverage verifies the precedence order.

Written by the indexing model from the issue text.