fix: validate taskRef.apiVersion format for custom tasks

[waveywaves]

Changes

add validation to ensure taskRef.apiVersion follows the correct format (group/version) when specified. Previously, invalid apiVersion values like 'invalid-api-version' were accepted, causing PipelineRuns to create unhandled CustomRuns that would timeout. Now Pipeline creation fails immediately with a clear error message when an invalid apiVersion is provided.

Fixes #9044

Submitter Checklist

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Release Notes

NONE

[tekton-robot]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1/pipeline_validation.go	99.3%	99.3%	0.0
pkg/apis/pipeline/v1beta1/pipeline_validation.go	99.0%	99.0%	0.0

[waveywaves]

/retest

[waveywaves]

/retest