[cherry-pick: release-v0.3.x] fix: selector-based pruning for label and annotation selectors

[tekton-robot]

This is a cherry-pick of #142

---

Changes

This PR fixes a bug in selector-based pruning where PipelineRuns were not being grouped correctly when using matchLabels or matchAnnotations in namespace-level ConfigMaps.

Problem

Label selectors: When a ConfigMap specified matchLabels: {env: test}, the pruner was using ALL labels from the PipelineRun to list resources, including Tekton-generated labels like tekton.dev/pipeline. This meant each PipelineRun was treated as its own unique group.

Annotation selectors: When a ConfigMap specified only matchAnnotations, the list selector became empty, causing all the resources in the namespace to be listed and the configuration to be applied incorrectly to unrelated PipelineRuns.

Solution

1. Added GetMatchingSelector() method to retrieve the exact selector from the ConfigMap that matched the resource
2. Modified doResourceCleanup() to use only the ConfigMap's matchLabels

/kind bug

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
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• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
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• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Selector-based pruning now correctly group PipelineRuns by ConfigMap-defined labels/annotations instead of all resource labels

[infernus01]

/kind misc

[infernus01]

/approve
/lgtm

[tekton-robot]

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[infernus01]

/remove-kind misc