[conftest]: Honor skip_check_dut_health marker in yang_validation_check

[sakshamkhurana21]

Description of PR

Make the autouse yang_validation_check module-scoped fixture in tests/conftest.py
honor the skip_check_dut_health marker, matching the existing precedent set by
core_dump_and_config_check in the same file.

Today, yang_validation_check runs unconditionally for every test module, including
maintenance / recovery / reboot / upgrade / HA modules that intentionally operate
against a transiently unreachable or unhealthy DUT. This causes flakes, see below:

autorestart/test_container_autorestart.py::test_containers_autorestart[vlab-03-None-bgp] ERROR [  5%]

==================================== ERRORS ====================================
_______ ERROR at setup of test_containers_autorestart[vlab-03-None-bgp] ________

item = <Function test_containers_autorestart[vlab-03-None-bgp]>

Summary:
Fixes Flaky test

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

Approach

What is the motivation for this PR?

test_posttest::test_restore_container_autorestart[vlab-03] (and several other
maintenance modules) is flaky on PR validation runs.

How did you do it?

Added an early-yield gate inside yang_validation_check, immediately after the
existing --skip_yang flag handling, that follows the same idiom used by
core_dump_and_config_check (tests/conftest.py:3117-3119):

for m in request.node.iter_markers():
    if m.name == "skip_check_dut_health":
        logger.info(
            "Skipping YANG validation: module marked skip_check_dut_health"
        )
        yield
        return

Any platform specific information?

N/a

Supported testbed topology if it's a new test case?

N/a

Documentation

N/a

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[auspham]

Approved as if the test is marked as skip_dut_health, we should tolerate yang validation as well.