fix: stop get_submission from changing process-wide warning filters

[Ardiea]

Description

get_submission() calls warnings.filterwarnings('error') before loading the submission and warnings.filterwarnings('default') in finally. It does this to turn jsonfield's invalid-JSON RuntimeWarning back into an exception, which jsonfield raised itself before 3.1.0.

warnings.filters is a single list shared by the whole process. Under a threaded WSGI server (we run Granian with 8 blocking threads per worker), this causes two problems:

1. Unrelated requests fail. While one thread is inside get_submission, any warning raised on another thread is raised as an exception. We see this in production as unhandled DeprecationWarnings (500s) from code that has nothing to do with submissions. For example, datetime.utcnow() in SessionInactivityTimeout.process_request fails on /event, /courseware-search/enabled/ and xblock handler routes.
2. The host's warning config is overridden. filterwarnings('default') puts a catch-all default filter at the front of the list, and it stays there after the first call. That cancels edx-platform's ignore rules in openedx/core/lib/logsettings.py (for example Setting _field_data is deprecated). The ('error', None, Warning, ...) entry also stays in the list, with only that default in front of it.

warnings.catch_warnings() would not fix this. Before Python 3.14 it also swaps the process-wide filter list.

Change

• Submission.answer now uses AnswerJSONField, a JSONField subclass. Its from_db_value returns invalid JSON as an InvalidJSONString (a JSONString subclass) and still emits jsonfield's RuntimeWarning. jsonfield wraps both valid JSON strings and invalid raw values in JSONString, so a separate marker type is needed to tell them apart.
• get_submission() checks for InvalidJSONString and raises SubmissionInternalError as before. It no longer touches warnings.filters.
• AnswerJSONField.deconstruct() returns jsonfield.fields.JSONField as the field path. No migration is generated (makemigrations --check is clean), so nothing alters the submissions table. On SQLite an AlterField here rebuilds the table.
• Version bumped to 4.0.3.

Answers are read and written the same way as before, and get_submission still uses one query.

Testing

• test_load_non_json_answer (existing) still passes: a corrupt raw_answer raises SubmissionInternalError from get_submission and get_submission_and_student.
• New test_get_submission_leaves_warning_filters_alone (string and dict answers) asserts that warnings.filters is the same after the call. It fails on master.
• New test_load_json_string_answer_that_looks_like_invalid_json: a valid JSON string answer '}' comes back as the answer and does not raise.
• Full suite: 221 passed. make test_quality (pylint, isort, pycodestyle) is clean.

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