Clear activeMounts on container stop

Applies memory leak prevention from this PR to the activeMounts feature
that was merged from main. When a container stops or restarts, mount
points become invalid but the Map still held stale references.
Clearing it maintains consistency with portTokens and defaultSession.

Author: ghostwriternr

packages/sandbox/src/sandbox.ts

@@ -764,6 +764,7 @@ export class Sandbox<Env = unknown> extends Container<Env> implements ISandbox {
     // This prevents stale references after container restarts
     this.portTokens.clear();
     this.defaultSession = null;
+    this.activeMounts.clear();
 
     // Persist cleanup to storage so state is clean on next container start
     await Promise.all([