fix: add docs-validate skip for Python snippet in streaming-events guide

The Python subscription example uses 'session' without defining it,
which fails mypy validation in CI. Add skip directive to match the
Go and .NET snippets.

Co-authored-by: Copilot <[email protected]>

Author: patniko

docs/guides/streaming-events.md

@@ -82,6 +82,7 @@ session.on("assistant.message_delta", (event) => {
 <details>
 <summary><strong>Python</strong></summary>
 
+<!-- docs-validate: skip -->
 ```python
 from copilot.generated.session_events import SessionEventType