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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.56%. Comparing base (717a04d) to head (d664de5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
+ Coverage   50.40%   50.56%   +0.16%     
==========================================
  Files          19       19              
  Lines        2113     2120       +7     
==========================================
+ Hits         1065     1072       +7     
  Misses        916      916              
  Partials      132      132              

Flag	Coverage Δ
unit-tests	50.56% <ø> (+0.16%)	⬆️

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