Fix the Python list_sessions docstring example to use session_id (#2099)

The Example block in CopilotClient.list_sessions printed
session.sessionId, but list_sessions returns list[SessionMetadata], and
that dataclass declares session_id. sessionId exists only as the wire
key, mapped to session_id inside SessionMetadata.from_dict/to_dict, so
anyone copying the documented example hit:

    AttributeError: 'SessionMetadata' object has no attribute 'sessionId'

Use the attribute the returned dataclass actually declares. The Go and
.NET bindings already document this method with their own idiomatic
spelling of the same field.

Docstring text only: no public API, behavior, or wire format change.

Co-authored-by: examon <examon@users.noreply.github.com>

Author: examon

python/copilot/client.py

@@ -3386,7 +3386,7 @@ async def list_sessions(self, filter: SessionListFilter | None = None) -> list[S
         Example:
             >>> sessions = await client.list_sessions()
             >>> for session in sessions:
-            ...     print(f"Session: {session.sessionId}")
+            ...     print(f"Session: {session.session_id}")
             >>> # Filter sessions by repository
             >>> from copilot.client import SessionListFilter
             >>> filtered = await client.list_sessions(SessionListFilter(repository="owner/repo"))