Overridding of pre/post definitions in tests requires manual flag

[deepsm007]

Add AllowPrePostStepOverrides to MultiStageTestConfiguration.
mergeWorkflow now validates pre/post overrides, requiring the flag set to true to prevent accidental resource leaks. Updated YAML docs and added comprehensive tests.

https://issues.redhat.com/browse/DPTP-4575

/cc @openshift/test-platform

[deepsm007]

/ai pr_description

[openshift-ci]

@deepsm007:
Add AllowPrePostStepOverrides to MultiStageTestConfiguration.
mergeWorkflow now validates pre/post overrides, requiring the flag set to true to prevent accidental resource leaks. Updated YAML docs and added comprehensive tests.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deepsm007

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[deepsm007]

/test images

[openshift-ci]

@deepsm007: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/breaking-changes	8d3f9a7	link	false	/test breaking-changes

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[droslean]

The filename is wrong. You need to put them in test_test.go