fix: preserve PipelineRun-owned TaskRuns during history pruning

[yuzichen12123]

Summary

History-limit cleanup can delete completed TaskRuns that belong to a still-running PipelineRun. The PipelineRun controller then treats those tasks as unscheduled and recreates them, causing completed pipeline work to run again.

This change excludes PipelineRun-owned resources from the history-limiter candidate set. It recognizes both ownership signals used by Tekton:

• the tekton.dev/pipelineRun label;
• a PipelineRun ownerReference.

The ownerReference check protects resources that do not carry the label.

Fixes #381

Testing

• go test ./...

The regression test verifies that history pruning removes eligible standalone resources while preserving PipelineRun children identified by either ownership mechanism.

[tekton-robot]

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