Changes

Added -o/--output flag to pipelinerun cancel and taskrun cancel
On success, when --output json or --output yaml is set, prints a structured result instead of the plain-text message: the examples are mentioned in the release notes

Default behavior (no --output flag) is unchanged — still prints PipelineRun cancelled:
Invalid --output values (e.g. csv) return a clear error

./tkn pipelinerun cancel sleep-pipeline-run-1789013716850-r-x6zw6 -n cancel-test -o csv 
Error: invalid output format "csv": must be json or yaml

Error paths (e.g. cancelling an already-finished run) still return plain error text regardless of --output, so failures are never mistaken for valid structured output

./tkn pipelinerun cancel sleep-pipeline-run-1789013716850-r-54f56 -n cancel-test -o yaml
Error: failed to cancel PipelineRun sleep-pipeline-run-1789013716850-r-54f56: PipelineRun has already finished execution

Reuses the shared pkg/formatted.PrintStructuredOutput/IsStructured helpers; introduces a CancelResult{Kind, Name, Status} struct local to each command package
Updated --help examples for both commands to demonstrate -o json/-o yaml usage

Submitter Checklist

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• Includes tests (if functionality changed/added)
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Release Notes

tkn pipelinerun cancel and tkn taskrun cancel now support --output json and --output yaml

Both commands accept a new -o/--output flag to print the cancellation result as structured JSON or YAML instead of plain text — useful for scripts, automation, and AI agents that need machine-readable output.

$ tkn pipelinerun cancel my-run -o json
{
    "kind": "PipelineRun",
    "name": "my-run",
    "status": "cancelled"
}

$ tkn taskrun cancel my-taskrun -o yaml
kind: TaskRun
name: my-taskrun
status: cancelled

Default (no --output flag) behavior is unchanged. An unsupported format (e.g. -o csv) returns a clear error.