refactor: store diff in action object (#1693)

[amnkarn]

Fixes #1693

Description

This PR refactors getDiff and related consumers to store and access the git diff directly on the action object (action.diff). This makes it easier for plugins and subsequent actions (like scanDiff) to retrieve the diff without needing to extract it from the step content.

Changes Made

• Backend: Updated getDiff to assign the diff string directly to action.diff and updated the Action interface.
• Processors: Updated scanDiff.ts to retrieve the diff from action.diff.
• UI: Updated PushActionView in types.ts to accept Step | string for the diff property.
• UI Services: Modified getPush in git-push.ts to handle the new string format.

Backward Compatibility

To ensure historical database records and existing UI components do not break, fallback logic has been added. If action.diff is not present (or is not a string), the system gracefully falls back to extracting the content from steps.find((s) => s.stepName === 'diff')?.content in both scanDiff.ts and git-push.ts.

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• ✅ login: amnkarn / name: Aman Karn (6a75417)
• ✅ login: amnkarn / name: Aman Kumar Karn (ed30c23)