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/lua_func/lua_func.lua

@@ -32,5 +32,13 @@ function module.tuple_return(first, second)
     return first, second
 end
 
+function module.set_context_value(val)
+    tm.setgd("_lua_ctx_test_value", val)
+    return val
+end
+
+function module.get_context_value()
+    return tm.gd("_lua_ctx_test_value")
+end
 
 return module