Add context_id to py_func and lua_func for shared persistent subprocess

- py_func and lua_func items accept a context_id parameter; items sharing
  the same id reuse the same subprocess for the duration of the test run
- Subprocess-side tm.setgd/tm.gd use a local fallback dict for non-JSON-
  serializable values (py_func only); serializable values reach the main
  process global dict and are accessible from any test item or subprocess
- Shared subprocess engines are cleaned up in process.py finally block
- LuaProcessBase gains is_alive() (was missing, broke all lua_func items)
- Validation tests cover serializable sharing across different context ids,
  non-serializable sharing within the same context_id, and cross-item access
- RST documentation updated for both py_func and lua_func items

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/validation/items/py_func/py_func.py

@@ -27,3 +27,23 @@ def echo(param):
 
 def tuple_return(first, second):
     return first, second
+
+def set_context_value(val):
+    tm.setgd("_py_ctx_test_value", val)
+    return val
+
+def get_context_value():
+    return tm.gd("_py_ctx_test_value", None)
+
+
+class _NotSerializable:
+    def __init__(self, val):
+        self.val = val
+
+def set_ns_value(val):
+    tm.setgd("_py_ctx_ns_value", _NotSerializable(val))
+    return val
+
+def get_ns_value():
+    obj = tm.gd("_py_ctx_ns_value", None)
+    return obj.val if obj is not None else None