Render content-identical renames using the previous file path

[Copilot]

Content-identical renames rendered as new files because base content was requested from the destination path, leaving the base pane empty.

• Base resolution
  • Load renamed files from previousFileName at the base commit.
  • Preserve current-path lookup for other change types.

const basePath = status === GitChangeType.RENAME
	? previousFileName
	: fileName;

• Regression coverage
  • Verify no-patch renames request base content from the original path.

• Fixes Renamed file with no content changes is shown as a 100% new file in the PR diff #8893

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[Copilot]

Pull request overview

Fixes rendering of content-identical renames in the PR diff by resolving base-side file content from the previous path (so the base pane isn’t empty and the rename doesn’t appear as a 100% new file).

Changes:

• Update GitFileChangeModel.showBase() to load base content for RENAME changes from previousFileName (for InMemFileChange / SlimFileChange) instead of the destination path.
• Add a regression test ensuring repository.show() is called with the previous filename for rename base content.
• Update the proposed VS Code chat API type definition for ChatResponseAutoModeResolutionPart (appears unrelated to the rename fix).

Show a summary per file

File	Description
src/view/fileChangeModel.ts	Resolves base-content lookup for renames using previousFileName when applicable.
src/test/view/fileChangeModel.test.ts	Adds regression coverage for base-content resolution on content-identical renames.
src/@types/vscode.proposed.chatParticipantAdditions.d.ts	Updates proposed API types/doc for auto-model routing resolution part.

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• Files reviewed: 2/3 changed files
• Comments generated: 0
• Review effort level: Lite