fix: freeze time to avoid jwt expiration

[wgu-taylor-payne]

Description

Fixes flaky ExpiredSignatureError failures in openedx/core/lib/tests/test_jwt.py by freezing time during test execution.

Root cause

test_now = int(time()) is evaluated once at module import time. Tokens are created with
exp = test_now + 1000. When unpack_and_verify calls jwt.decode, PyJWT checks exp against
the real wall clock via time.time(). If the test runs more than 1000 seconds after the module
was imported (due to test ordering or slow CI), the token is already expired.

Fix

Wrap both test classes with @freeze_time at the test_now timestamp so that time.time()
returns a consistent value during test execution. This ensures PyJWT's internal exp check
sees the same clock that was used to create the token.

Also converts assertions to pytest style (assert ==, pytest.raises) and removes the
corresponding # noqa suppressions.

Also removes unused variables (expected_token_with_claims) in a couple of methods.

No production code changes.

Supporting information

• Flaky tests in openedx/core/lib/tests/test_jwt.py tests with jwt.exceptions.ExpiredSignatureError #38450
• Failing CI log

Testing instructions

1. Run the test file:

   pytest openedx/core/lib/tests/test_jwt.py --ds=lms.envs.test --no-migrations -x -v
   

2. All 10 tests should pass.

Deadline

None

Other information

• No dependencies on other changes.
• freezegun is already in test requirements (requirements/edx/testing.in).
• Used Kiro (AI assistant) to help develop this change. Kiro wrote the base implementation, ran tests/linting. I reviewed the approach, made some edits (comments, naming), directed the assertion style conversion to pytest, chose the decorator ordering, and verified the fix didn't drop any meaningful test coverage.

[openedx-webhooks]

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[robrap]

Thanks!