fix: update callback return value

[CarolYeh910]

Release Summary:

A successful cert validation callback should return only S2N_SUCCESS. Previously, both 0 and any positive return value were considered successful.

Description of changes:

This is a follow-up change on PR #5110

Per our documentation, s2n_cert_validation_callback should return S2N_SUCCESS on success. However, the old behavior accepted both 0 and positive values. This PR aims to fix this inconsistency.

Testing:

Confirmed s2n_cert_validation_callback_test passed with this change

$ cmake --build build/ --target test -- ARGS="-L unit -R s2n_cert_validation_callback_test --output-on-failure"
Running tests...
Test project /home/ubuntu/s2n-tls/build
    Start 18: s2n_cert_validation_callback_test
1/1 Test #18: s2n_cert_validation_callback_test ...   Passed    0.74 sec

100% tests passed, 0 tests failed out of 1

Label Time Summary:
unit    =   0.74 sec*proc (1 test)

Total Test time (real) =   0.74 sec

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.