Add comprehensive test coverage for SettingsForm class

[devin-ai-integration]

Summary

Adds 14 new unit tests for the SettingsForm class in realworld/accounts/tests.py. The tests cover form validation, save behavior with and without password changes, field validation, and optional field handling.

Test coverage includes:

• Valid form submission with and without password
• Saving user settings while preserving existing password
• Saving user settings with a new password
• Validation errors for invalid email, invalid image URL, empty name, and empty email
• commit=False behavior verification
• Optional fields (bio, image) can be blank
• Form field presence and password field not being required

All 47 tests pass locally (33 existing + 14 new).

Review & Testing Checklist for Human

• Verify test isolation: tests use setUpTestData which shares a user instance - confirm tests that call form.save() don't cause flaky behavior when run in different orders
• Run python manage.py test realworld.accounts.tests.TestSettingsForm -v 2 to verify all new tests pass

Notes

Link to Devin run: https://app.devin.ai/sessions/36d195ba1f39460d85730799a6171c41
Requested by: shayan@cognition.ai

[devin-ai-integration]

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