Document code context isolation guarantees

Add documentation explaining that each code context is completely
isolated with a dedicated executor process. Includes example showing
concurrent execution across multiple contexts.

Related to cloudflare/sandbox-sdk#249 which fixed state isolation bug.

Author: github-actions[bot]

src/content/docs/sandbox/api/interpreter.mdx

@@ -37,6 +37,10 @@ const ctx = await sandbox.createCodeContext({
 ```
 </TypeScriptExample>
 
+:::note[Context isolation]
+Each code context is completely isolated from other contexts. Variables, imports, functions, and state defined in one context are never accessible to another context. Each context has a dedicated executor process from creation to deletion, ensuring complete isolation even under heavy concurrent usage.
+:::
+
 ### `runCode()`
 
 Execute code in a context and return the complete result.
@@ -70,6 +74,13 @@ await sandbox.runCode('import math; radius = 5', { context: ctx });
 const result = await sandbox.runCode('math.pi * radius ** 2', { context: ctx });
 
 console.log(result.results[0].text); // "78.53981633974483"
+
+// Create isolated contexts for concurrent execution
+const ctx2 = await sandbox.createCodeContext({ language: 'python' });
+await Promise.all([
+  sandbox.runCode('x = 1', { context: ctx }),
+  sandbox.runCode('x = 2', { context: ctx2 })
+]);
 ```
 </TypeScriptExample>