fix: prevent race conditions in McpServerManager singleton initialization

Added thread-safe initialization to the McpServerManager singleton pattern to
prevent potential race conditions when getInstance is called concurrently.
The changes include:

1. Added initializationPromise to track ongoing initialization
2. Implemented double-checked locking pattern:
   - First check: Return existing instance if available
   - Second check: Wait for any ongoing initialization
   - Third check: Double-check inside initialization block
3. Added proper cleanup in finally block to prevent deadlocks

This ensures that:
- Only one McpHub instance is ever created
- Concurrent calls wait for initialization to complete
- Resources are properly initialized and cleaned up
- No memory leaks from incomplete initialization

The fix maintains the existing functionality while making it safe for
concurrent access in VS Code's multi-window environment.

src/services/mcp/McpServerManager.ts

@@ -10,21 +10,44 @@ export class McpServerManager {
 	private static instance: McpHub | null = null
 	private static readonly GLOBAL_STATE_KEY = "mcpHubInstanceId"
 	private static providers: Set<ClineProvider> = new Set()
+	private static initializationPromise: Promise<McpHub> | null = null
 
 	/**
 	 * Get the singleton McpHub instance.
 	 * Creates a new instance if one doesn't exist.
+	 * Thread-safe implementation using a promise-based lock.
 	 */
 	static async getInstance(context: vscode.ExtensionContext, provider: ClineProvider): Promise<McpHub> {
 		// Register the provider
 		this.providers.add(provider)
 
-		if (!this.instance) {
-			this.instance = new McpHub(provider)
-			// Store a unique identifier in global state to track the primary instance
-			await context.globalState.update(this.GLOBAL_STATE_KEY, Date.now().toString())
+		// If we already have an instance, return it
+		if (this.instance) {
+			return this.instance
 		}
-		return this.instance
+
+		// If initialization is in progress, wait for it
+		if (this.initializationPromise) {
+			return this.initializationPromise
+		}
+
+		// Create a new initialization promise
+		this.initializationPromise = (async () => {
+			try {
+				// Double-check instance in case it was created while we were waiting
+				if (!this.instance) {
+					this.instance = new McpHub(provider)
+					// Store a unique identifier in global state to track the primary instance
+					await context.globalState.update(this.GLOBAL_STATE_KEY, Date.now().toString())
+				}
+				return this.instance
+			} finally {
+				// Clear the initialization promise after completion or error
+				this.initializationPromise = null
+			}
+		})()
+
+		return this.initializationPromise
 	}
 
 	/**