Add comprehensive test coverage for SettingsForm class

[devin-ai-integration]

Summary

This PR adds comprehensive test coverage for the SettingsForm class in the accounts app. The SettingsForm is used for updating user profile settings (name, email, bio, image, and optional password change) but previously had no dedicated tests.

12 new tests are added covering:

• Form validation with valid data (all fields and required fields only)
• Form validation with invalid data (missing name, missing email, invalid email format, invalid image URL)
• Save behavior without password change (preserves existing password)
• Save behavior with password change (updates password correctly)
• Save behavior with commit=False (doesn't persist to database)
• Form field presence and password field optionality

Review & Testing Checklist for Human

• Verify the tests align with the actual SettingsForm implementation in realworld/accounts/forms.py - ensure all form fields and save logic are covered
• Run the full test suite locally to confirm all 25 accounts tests pass: python manage.py test realworld.accounts

Notes

• All tests passed locally (12 new + 13 existing = 25 total)
• Tests follow the existing patterns in the codebase (using setUpTestData, similar assertion styles)
• No production code was modified

Link to Devin run: https://app.devin.ai/sessions/ce540827d2dd4c0a9a0adf652431c69b
Requested by: Angela Lin (angela.lin@cognition.ai) / @angelalincog

[devin-ai-integration]

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