Migrate from `is_context_in_segment` to `get_evaluation_result`

[gagantrivedi]

As part of the flag-engine v10 migration (PR #6653), we vendored is_context_in_segment from the fix/missing-export branch. However, is_context_in_segment is not part of flag-engine's public API anymore.

Task

Switch from is_context_in_segment to get_evaluation_result.

Related

• Parent issue: Context Values: Bump flagsmith-flag-engine to ^10.0.3. #5991
• Phase 1 PR: feat: upgrade flagsmith-flag-engine to v10 #6653
• Phase 2 issue: Migrate to native flag-engine v10 APIs and remove vendored models #6654
• Review comment: feat: upgrade flagsmith-flag-engine to v10 #6653 (comment)