feat: apply AI generated code and fix errors in code using AI

[akshatbatra]

Closes #375

Features to apply AI suggested code and fix errors in that code with the help of AI.

Changes

• Apply code button now appears on the code blocks in chat panel that can be applied to the editors.
• CodeDiffPopup shows an interface to the user to select what code to accept/reject, click or drag on lines to toggle their addition/removal state.
• Whenever an error shows up, "AI Fix" button shows up which sends the error message in the chat window for AI to come up with a solution.

Flags

• The quality of the code depends on the model that you are using, it is recommended to use models like Qwen3 Coder, Claude Sonnet 4 and GPT-5 etc.
• This work was done with the old Playground UI as the base, it was later rebased onto restructuring/redesign branch to allow for reviewing with new design. When the redesign branch was merged, this branch was rebased onto main.

Screenshots or Video

ai-apply-fix-code-demo.webm

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

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