Add tests for SROS2 access control policies (Issue #238)

[Aaravanand00]

Description

This PR adds negative testing for SROS2 access control policies by ensuring that nodes configured with restricted security policies (DENY rules) correctly fail to communicate.

Key changes include :-

• Created two new security policies (policy_publisher_denied.xml and policy_subscriber_denied.xml) that restrict publishing and subscribing on the chatter topic.
• Updated sros_artifacts.cmake to generate cryptographic artifacts for the new /publisher_denied and /subscriber_denied enclaves.
• Refactored test_secure_publisher.cpp and test_secure_subscriber.cpp to dynamically parse their enclave names from argv rather than hardcoding them.
• Updated test_secure_publisher_subscriber.py.in to explicitly pass --ros-args --enclave to both nodes.
• Added a new access_control_tests matrix in CMakeLists.txt that pairs authorized nodes with denied nodes and asserts that the subscriber gracefully times out.

Fixes #238

Is this user-facing behavior change?

No, these changes are strictly related to internal testing.

Did you use Generative AI?

Yes ai agent was used to assist in scaffolding the test implementations and CMake loops.

Additional Information

The tests have been verified locally (with SECURITY=ON) and all security/access-control related test timeouts function correctly, ensuring that invalid access attempts are blocked at the middleware level.

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[Aaravanand00]

@fujitatomoya ptal