When validating objects created via Python constructor (not from
user JSON), set direct_construction flag so schemas can allow
optional fields like returnValueTest to be None.

Update ConditionVariable.Schema to set in_condition_variable flag
during deserialization, and update _ConditionField to propagate
this context to nested condition schemas. This allows conditions
inside ConditionVariable to have optional returnValueTest.

Also update ExecutionCallCondition to make returnValueTest optional
when inside a ConditionVariable or when directly constructed.

Update ContextVariableCondition, JsonCondition, and
SigningObjectAttributeCondition to allow returnValueTest to be
omitted when inside a ConditionVariable or when directly constructed.

When returnValueTest is None, verify() returns (True, value),
treating successful extraction as a passing condition.

Update RPCCondition, ContractCondition, TimeCondition, and
BaseJsonRequestCondition to handle None returnValueTest:
- Skip return type validation when returnValueTest is None
- Return (True, result) from verify() when no test is defined
- Update TimeCondition repr and timestamp property for None case

Test that conditions inside ConditionVariable can omit returnValueTest
for pure value extraction, while standalone conditions still require it.
Covers JsonCondition, ContextVariableCondition, SequentialCondition,
operations, and serialization roundtrips.

Convert unittest-style test class to pytest standard format to match
the rest of the test file. Moved inline imports to top of file.
Fixed incorrect assertion in test_json_condition_without_return_value_test_with_operations.

- test_context_var_condition.py: Update to reflect that direct
  construction without returnValueTest is now allowed, but
  from_dict (standalone) still requires it
- test_optional_return_value_test.py: Fix invalid string values
  in ReturnValueTest that don't pass ast.literal_eval