Add exhaustive pytest test suite with 169 tests covering all modules

Tests cover: ResponseService (response planning, fact filtering, PID
tracking, operation lifecycle), ProcessNode/ProcessTree (tree operations,
parent/child traversal, PID-to-GUID mapping, store dedup), all 8 parsers
(basic_strip, childprocess, ecs_sysmon, key_value, ports, process,
processguids, sysmon), all 4 requirements (base_requirement, basic,
has_property, source_fact), and hook.py (metadata, enable, expansion).

Includes pytest.ini, tox.ini, and a conftest.py with lightweight framework
stubs enabling isolated testing without the full Caldera installation.

Author: deacon-mp

pytest.ini

@@ -0,0 +1,5 @@
+[pytest]
+testpaths = tests
+asyncio_mode = auto
+markers =
+    asyncio: mark a test as an asyncio coroutine