Fix Ruff E501 line-length in preamble E2E test docstring

The new preamble test docstring exceeded the 100-char limit, failing
'ruff check' in the Python SDK Tests CI job across all platforms.

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

Author: MackinnonBuck

python/e2e/test_system_message_sections_e2e.py

@@ -44,7 +44,7 @@ async def test_should_use_replaced_identity_section_in_response(self, ctx: E2ETe
         await session.disconnect()
 
     async def test_should_use_replaced_preamble_section_in_response(self, ctx: E2ETestContext):
-        """Test that replacing only the preamble section causes the assistant to adopt a new persona"""
+        """Test that replacing only the preamble section changes the assistant persona"""
         session = await ctx.client.create_session(
             system_message={
                 "mode": "customize",