[CX_HARDENING] Enhance DA Task Test Coverage

[shamb0]

This PR:

This PR partially addresses issue #3160 by adding test cases for the following scenarios:

• Handling outdated proposals.
• Detecting duplicate votes.
• Validating vote collection and processing.
• Ensuring non-leader nodes correctly handle DA votes.

Key Changes

1. Added a new test file: crates/testing/tests/tests_cx_hardening/cxh_da_task.rs.
2. Introduced the create_lean_test_handle() function to create a lightweight system handle for task state testing.
3. Fixed a bug in the build_da_cert<> function located in HotShot/crates/testing/src/helpers.rs.
4. Enhanced test functionality to verify external events on output_event_stream.

This PR does not:

• Introduce any other changes beyond the mentioned test cases and bug fixes.

Key Areas to Review:

1. All files committed as part of this PR.