Fix to exclude creating slackreportconfig for excluded pattern

[deepsm007]

Follow up of #4758
/cc @openshift/test-platform

[openshift-ci]

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[deepsm007]

/ai pr_description

[openshift-ci]

@deepsm007:
Add full‑job‑name based Slack config lookup and drop default merging

• Introduce GetSlackReporterConfigForJobName that checks excluded_job_patterns against full job names and respects JobName/JobNamePatterns.
• Update presubmit, postsubmit, periodic, and images generation to call this new method.
• Remove legacy Slack config setting from jobbase.go; Slack is now configured per job during generation.
• Add tests:
  • MergeDefaults now never merges SlackReporterConfigs.
  • Jobs excluded by patterns no longer get SlackReporterConfig.
• Update fixtures to match new output.
• No other behavior changes.

[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	cdbfe80	link	false	/test breaking-changes

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[deepsm007]

/test images