Document code context isolation improvements

Adds documentation for improved code context isolation:
- Each context now gets a dedicated executor process (1:1 binding)
- Contexts prevent concurrent execution on the same context
- Complete state isolation between contexts guaranteed

Related to cloudflare/sandbox-sdk#249

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Co-Authored-By: Claude <[email protected]>

Author: github-actions[bot]

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

@@ -28,6 +28,8 @@ const context = await sandbox.createCodeContext(options?: CreateContextOptions):
 
 **Returns**: `Promise<CodeContext>` with `id`, `language`, `cwd`, `createdAt`, `lastUsed`
 
+**Isolation guarantees**: Each code context gets a dedicated executor process from creation to deletion, ensuring complete state isolation between contexts. Variables, imports, and functions defined in one context never leak to another context.
+
 <TypeScriptExample>
 ```
 const ctx = await sandbox.createCodeContext({
@@ -146,6 +148,24 @@ await sandbox.deleteCodeContext(ctx.id);
 ```
 </TypeScriptExample>
 
+:::note[Concurrent execution on same context]
+A code context can only execute one piece of code at a time. If you attempt to run code on a context that is already executing, the request will fail with an error. To run code concurrently, create multiple contexts:
+
+<TypeScriptExample>
+```
+// Create separate contexts for concurrent execution
+const ctx1 = await sandbox.createCodeContext({ language: 'python' });
+const ctx2 = await sandbox.createCodeContext({ language: 'python' });
+
+// Run code concurrently in different contexts
+const results = await Promise.all([
+  sandbox.runCode('import time; time.sleep(2); 42', { context: ctx1 }),
+  sandbox.runCode('import time; time.sleep(2); 100', { context: ctx2 })
+]);
+```
+</TypeScriptExample>
+:::
+
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