Fix(MockCA, Batch): Fix PVNO Setting Batch PKIMessage Responses

[Guiliano99]

Fix the NestedHandler so that responses to nested and batch PKIMessages
correctly mirror the sender's pvno value, as required by RFC 9483 Section 3.1.
Add Robot Framework and unit tests to verify the behaviour across all relevant scenarios.

Description

• Pass pvno=request["header"]["pvno"] when constructing the response PKIMessage
  in NestedHandler.handle_nested_message() so the response reflects the
  sender's protocol version.
• Add Robot Framework test CA MUST Respond with A Correct PVNO For Inner Added Protection verifying that an added-protection nested message with pvno=3
  receives a response with pvno=3.
• Add Robot Framework tests for batch messages: outer pvno=3 / inner pvno=2,
  reversed (pvno=2 / pvno=3), and mixed inner pvno values (2, 3, 2).
• Extend _generate_nested_message() in test_process_batch_message.py with
  explicit pvno values and add assertions that both the outer response and
  individual inner responses carry the correct pvno.

Motivation and Context

• Fix implementation.
• Added missing test cases.

How Has This Been Tested?

• Run the Robot Framework test suite (tests/pki_mgmt_entity_op.robot) covering
  added-protection nested messages and batch messages with uniform, reversed, and
  mixed pvno configurations.