fix(sessions): Order database sessions deterministically

[Sehlani042]

Summary

• order DatabaseSessionService.list_sessions() by create_time ASC with stable user_id / id tie-breakers
• add a regression test where database insertion order differs from session creation time order

Why

list_sessions() previously executed without an ORDER BY, so consumers using sessions[-1] for the newest session could receive an older row depending on database heap order.

Fixes #6272

Testing

• RED before fix: uv run pytest tests/unittests/sessions/test_session_service.py::test_database_session_service_list_sessions_orders_by_create_time_then_id -q failed with unordered ids
• GREEN after fix: uv run pytest tests/unittests/sessions/test_session_service.py::test_database_session_service_list_sessions_orders_by_create_time_then_id -q
• uv run pytest tests/unittests/sessions/test_session_service.py::test_create_and_list_sessions tests/unittests/sessions/test_session_service.py::test_list_sessions_all_users tests/unittests/sessions/test_session_service.py::test_database_session_service_list_sessions_orders_by_create_time_then_id -q
• uv run pytest tests/unittests/sessions/test_session_service.py -q
• uv run --extra dev pyink --check src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py
• uv run --extra dev ruff check src/google/adk/sessions/database_session_service.py tests/unittests/sessions/test_session_service.py
• git diff --check HEAD~1..HEAD

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