fix: prevent TextEditor disposed warnings by adding proper disposal checks

[roomote]

Fixes #5954

Problem

The extension was generating frequent "TextEditor is closed/disposed" warnings (tens per second) during API actions. This was caused by DecorationController and DiffViewProvider attempting to access TextEditor instances after they had been disposed.

Solution

• Added isDisposed flag and dispose() method to DecorationController to track disposal state
• Added isEditorValid() checks before accessing any TextEditor methods
• Properly dispose decoration controllers in DiffViewProvider.reset() to clean up resources
• Handle test environments where visibleTextEditors may be empty

Changes

• DecorationController.ts: Added disposal tracking and validation checks before all editor operations
• DiffViewProvider.ts: Added editor validation checks and proper cleanup of decoration controllers

Testing

• All existing tests pass
• The changes handle both production and test environments correctly

This ensures that once an editor is closed/disposed, it is not accessed again, eliminating the warning spam.

---

Important

Adds disposal checks and validation for TextEditor instances in DecorationController.ts and DiffViewProvider.ts to prevent frequent warnings.

• Behavior:
  • Adds isDisposed flag and dispose() method to DecorationController to track disposal state.
  • Adds isEditorValid() checks in DecorationController and DiffViewProvider to validate TextEditor instances before operations.
  • Properly disposes decoration controllers in DiffViewProvider.reset() to clean up resources.
• Files:
  • DecorationController.ts: Implements disposal tracking and validation checks before editor operations.
  • DiffViewProvider.ts: Adds editor validation checks and ensures proper cleanup of decoration controllers.
• Testing:
  • All existing tests pass, ensuring changes handle both production and test environments correctly.

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[daniel-lxs]

I checked the VSCode and our code and this doesn't seem to have an impact on Roo Code https://github.com/microsoft/vscode/blob/90072b47cf68321a916c4fbb04ed927b01e50af4/src/vs/workbench/api/common/extHostTextEditor.ts#L679

It only happens when these are called:
src/integrations/editor/DiffViewProvider.ts:558-561

this.activeDiffEditor.revealRange(
				new vscode.Range(scrollLine, 0, scrollLine, 0),
				vscode.TextEditorRevealType.InCenter,
			)

src/integrations/editor/DiffViewProvider.ts:578-581

				this.activeDiffEditor.revealRange(
					new vscode.Range(lineCount, 0, lineCount, 0),
					vscode.TextEditorRevealType.InCenter,
				)

Closing for now unless I'm wrong and this affects some critical functionality.