Fix: disposed ChatThinkingContentPart returned by getLastThinkingPart causing DisposableStore error (#296927)

* Initial plan

* fix: set isActive=false on ChatThinkingContentPart dispose to prevent DisposableStore error

Co-authored-by: jrieken <1794099+jrieken@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jrieken <1794099+jrieken@users.noreply.github.com>

Author: Copilot

src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatThinkingContentPart.ts

@@ -1526,6 +1526,7 @@ ${this.hookCount > 0 ? `EXAMPLES WITH BLOCKED CONTENT (from hooks):
 	}
 
 	override dispose(): void {
+		this.isActive = false;
 		if (this.markdownResult) {
 			this.markdownResult.dispose();
 			this.markdownResult = undefined;

src/vs/workbench/contrib/chat/test/browser/widget/chatContentParts/chatThinkingContentPart.test.ts

@@ -983,6 +983,25 @@ suite('ChatThinkingContentPart', () => {
 			assert.strictEqual(part.getIsActive(), false, 'Should be inactive after markAsInactive');
 		});
 
+		test('dispose should set isActive to false', () => {
+			const content = createThinkingPart('**Active thinking**');
+			const context = createMockRenderContext(false);
+
+			const part = instantiationService.createInstance(
+				ChatThinkingContentPart,
+				content,
+				context,
+				mockMarkdownRenderer,
+				false
+			);
+
+			assert.strictEqual(part.getIsActive(), true, 'Should start as active');
+
+			part.dispose();
+
+			assert.strictEqual(part.getIsActive(), false, 'Should be inactive after dispose');
+		});
+
 		test('collapseContent should collapse the part', () => {
 			const content = createThinkingPart('**Content**\nSome detailed reasoning that differs from the title');
 			const context = createMockRenderContext(false);