fix: filter NUL characters from chat records

[XIIRUAN]

What this PR does / why we need it?

PostgreSQL rejects native NUL (0x00) characters in text fields. Workflow output can reach ChatRecord.problem_text or ChatRecord.answer_text and cause the record write to fail.

Related to #6722.

Summary of your change

• Remove NUL characters from chat record question and answer text before persistence.
• Keep the fix scoped to ChatRecord instead of changing every model.
• Add a regression test that exercises the real ChatRecord.save() sanitization path.

Validation

• ChatRecordTestCase.test_save_removes_nul_characters
• ruff check apps/application/models/application_chat.py apps/application/tests.py
• Python compilation and diff checks

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact; no documentation change is required for this fix.