Preserve PR diff scroll position

[raashish1601]

Fixes #8206.

Summary:

• Preserve the visible range when re-selecting a PR diff from the tree.
• Restore that range when the diff reopens so the scroll position stays stable.

Validation:

• npm run compile:test
• npm run compile:node
• npx eslint src/commands.ts src/view/treeNodes/fileChangeNode.ts src/test/view/fileChangeNode.test.ts
• npm test could not complete here because the Windows VS Code Insiders binary rejected the extension-host CLI flags in this environment.

Manual risk:

• The fix is validated by compile/lint coverage, but a quick repro in VS Code Insiders is still the best final confirmation for the extension-host behavior.