Fix Python subagent hooks test to scope env var to client instance

Instead of polluting global os.environ at module import time, create a
dedicated client with the feature flag in its env dict. This matches how
the .NET and Go tests scope the env var to the client instance.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

python/e2e/test_subagent_hooks_e2e.py

@@ -5,10 +5,9 @@
 
 import os
 
-os.environ["COPILOT_EXP_COPILOT_CLI_SESSION_BASED_SUBAGENTS"] = "true"
-
 import pytest
 
+from copilot.client import CopilotClient, SubprocessConfig
 from copilot.session import PermissionHandler
 
 from .testharness import E2ETestContext
@@ -40,7 +39,22 @@ async def on_post_tool_use(input_data, invocation):
             })
             return None
 
-        session = await ctx.client.create_session(
+        # Create a client with the session-based subagents feature flag
+        env = ctx.get_env()
+        env["COPILOT_EXP_COPILOT_CLI_SESSION_BASED_SUBAGENTS"] = "true"
+        github_token = (
+            "fake-token-for-e2e-tests" if os.environ.get("GITHUB_ACTIONS") == "true" else None
+        )
+        client = CopilotClient(
+            SubprocessConfig(
+                cli_path=ctx.cli_path,
+                cwd=ctx.work_dir,
+                env=env,
+                github_token=github_token,
+            )
+        )
+
+        session = await client.create_session(
             on_permission_request=PermissionHandler.approve_all,
             hooks={
                 "on_pre_tool_use": on_pre_tool_use,
@@ -73,3 +87,4 @@ async def on_post_tool_use(input_data, invocation):
         )
 
         await session.disconnect()
+        await client.stop()