Fix:Align Core API feature_segment shape with Edge API

[SahilJat]

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• [ x ] I have filled in the "How did you test this code" section below.

Changes

Closes #7456

• api/features/serializers.py:
  • Introduced _FeatureSegmentPrioritySerializer to handle the nested
    serialization of the priority field for feature segments.
  • Updated FeatureStateSerializerFull to use this new serializer for
    the feature_segment field.
  • This change ensures that the Core API's SDK-facing endpoints (and
    any other endpoints using FeatureStateSerializerFull) return the
    same data shape as the Edge API, providing the priority information
    that was previously missing.
• api/tests/unit/environments/identities/test_unit_identities_views.py:
  • Added a new test case

How did you test this code?

Automated Tests: Added a new unit test in
api/tests/unit/environments/identities/test_unit_identities_views.py that specifically sets up a segment override with a defined priority
and asserts that the /api/v1/identities/ response contains the
feature_segment as an object: {"priority": 1}.

• Manual Verification: Analyzed the serializer inheritance chain to
  ensure the change correctly propagates to the SDK identities and flags
  endpoints while minimizing impact on write operations (as the field is
  marked as read_only=True in the "Full" serializer).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (6168392) to head (27f5ba9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7523      +/-   ##
==========================================
- Coverage   98.47%   98.31%   -0.16%     
==========================================
  Files        1400     1400              
  Lines       52980    52873     -107     
==========================================
- Hits        52170    51981     -189     
- Misses        810      892      +82     

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