Add comprehensive test coverage for SettingsForm component

Summary

This PR adds comprehensive test coverage for the SettingsForm component that was previously missing from the test suite. The implementation includes:

• TestSettingsForm class (17 tests): Covers form validation, password handling, model integration, and edge cases
• TestSettingsViewIntegration class (4 tests): Tests integration with the settings view and HTMX functionality

The tests follow existing patterns in the codebase using TestCase with setUpTestData() and cover all specified requirements including:

• Form validation tests (email formats, required fields, field length constraints)
• Password handling (hashing when provided, preservation when empty, whitespace handling)
• Model integration (data persistence, unique constraints, instance updates)
• Edge cases (commit=False, field ordering, form initialization)
• View integration (GET/POST requests, HTMX error responses)

All 34 tests in the accounts app now pass, confirming no regressions were introduced.

Review & Testing Checklist for Human

• Verify form validation behavior: Test the actual settings form in the app to ensure the test assertions match real form behavior, especially for email validation and field constraints
• Test password handling security: Manually verify that password updates work correctly and that empty passwords preserve existing ones (critical security behavior)
• Run full test suite: Confirm all tests pass consistently and no flaky behavior exists in the new tests

Notes

The initial test implementation had some failing tests that required adjustment to match Django's actual behavior rather than assumed behavior (URLField validation, whitespace password handling, HttpResponseClientRedirect status codes). This suggests the test assertions needed refinement to accurately reflect the real form behavior.

Link to Devin run: https://app.devin.ai/sessions/f8f88bd987024cf08a65708bdd8ea7cb
Requested by: Jude Partovi (@jpartovi)