feat(debug): enable Python debug workflow and debug server integration

Adds an end-to-end Python debug workflow for the extension.
Includes a new prepare-debug.ps1 script and a VS Code task.
Extends the Python server and extension to coordinate a debug session and safe startup.

- Add prepare-debug.ps1 and a VS Code task to build the debug bundle
- Enable Python debug wiring in the server and tests
- Ensure a single stable Python debug session during startup

client/src/extension.ts

@@ -38,6 +38,7 @@ export async function activate(context: vscode.ExtensionContext) {
     const serverInfo = loadServerDefaults()
     const serverName = serverInfo.name
     const serverId = serverInfo.module
+    const pythonDebugMode = process.env.USE_DEBUGPY?.toLowerCase() === "true"
 
     // Setup logging
     const outputChannel = createOutputChannel(serverName)
@@ -101,10 +102,15 @@ export async function activate(context: vscode.ExtensionContext) {
         return runServerQueue
     }
 
+    if (!pythonDebugMode) {
+        context.subscriptions.push(
+            onDidChangePythonInterpreter(async () => {
+                await runServer()
+            })
+        )
+    }
+
     context.subscriptions.push(
-        onDidChangePythonInterpreter(async () => {
-            await runServer()
-        }),
         onDidChangeConfiguration(async (e: vscode.ConfigurationChangeEvent) => {
             if (checkIfConfigurationChanged(e, serverId)) {
                 await runServer()
@@ -116,6 +122,15 @@ export async function activate(context: vscode.ExtensionContext) {
     )
 
     setImmediate(async () => {
+        if (pythonDebugMode) {
+            // A debugpy listen session is attached to exactly one process. Do not
+            // subscribe to interpreter changes during startup, as the Python
+            // extension can emit a duplicate event and restart that process.
+            traceLog("Python debug mode: starting one stable server session")
+            await runServer()
+            return
+        }
+
         const interpreter = getInterpreterFromSetting(serverId)
         if (interpreter === undefined || interpreter.length === 0) {
             traceLog(`Python extension loading`)