fix(customrun): return error when customrun is not found during delete

[shubhamrai9122-creator]

Changes

Fixes #3329

Previously, deleteCustomRuns printed a 'not found' message when customRunExists returned an error, but skipped the resource without recording the error into deleteErr. As a result, tkn customrun delete exited 0 even when specified resources were not found.

This commit records lookup errors using multierr.Append to align CustomRun delete behavior with Task and Pipeline delete:

• Missing names still print the message and are skipped.
• Existing names are still deleted and reported.
• The command returns an error if any name was not found.

Submitter Checklist

• Includes tests
• Commit messages follow commit message best practices

Release Notes

Fix tkn customrun delete to return a non-zero exit code when a specified customrun is not found.

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[linux-foundation-easycla]

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• ✅ login: shubhamrai9122-creator / name: shubhamrai9122-creator (6838250)

[pratap0007]

@shubhamrai9122-creator Thank you for the contribution, could you please sign the CLA?

[Copilot]

🟢 Approval recommended

The focused fix preserves successful results, propagates failures, and includes regression coverage with no unresolved findings.

No open issues 🎉

What changed in this PR

Fixes #3329 by making tkn customrun delete return an error when a requested resource is missing, while preserving successful deletions.

Changes:

• Accumulates lookup and deletion errors.
• Strengthens error assertions and adds regression coverage for mixed successful and missing resources.

File	Description
pkg/​cmd/​customrun/​delete.go	Collects errors without stopping successful deletions.
pkg/​cmd/​customrun/​delete_test.go	Updates error expectations and tests partial success with JSON output.

_{Review effort: Balanced}

[shubhamrai9122-creator]

@pratap0007 Thanks for the review! I've signed the CLA, and the EasyCLA check is passing now. The PR is ready for another look whenever you have time.