fix(features): return 400 instead of 500 on feature name race condition

[narendraio]

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Changes

Closes #6617

When two requests create a feature with the same name concurrently, both can
pass validate_name before either is persisted. The second insert then trips
the case-insensitive lowercase_feature_name unique constraint, raising an
unhandled IntegrityError that surfaces to the client as an HTTP 500.

This catches the IntegrityError during creation and re-raises it as a
validation error, so the client receives a clear 400 response consistent with
the existing duplicate-name validation. The duplicate-name message is extracted
into a shared constant so both code paths stay in sync.

How did you test this code?

Added a unit test (test_create_feature__name_race_condition__returns_400) that
simulates the race by letting validate_name pass while the database still
rejects the duplicate name, and asserts the endpoint responds with a 400 and the
expected error message. Verified locally against Postgres:

make test opts='tests/unit/features/test_unit_features_views.py -k name_race_condition'

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