Improve input validation for FASTQ

[scwatts]

Description of feature

Validation of input FASTQ files should be improved:

• certain unsupported samplesheet FASTQ input rows produce non-descript errors
  • see here for an example: https://nfcore.slack.com/archives/C04CQ1LK7U1/p1724454431405079
• all file inputs including FASTQ should be init as Nextflow.File objects and checked to exist (w/ stub mode exception):

oncoanalyser/lib/Utils.groovy

Lines 127 to 133 in 4227bc1

	meta_sample[filetype_enum][fastq_key] = ['fwd': fwd, 'rev': rev]
	} else {
	meta_sample[filetype_enum] = Utils.getFileObject(it.filepath)

---

This work may also relate to:

• Migrate from nf-validation to nf-schema #51

[scwatts]

Additionally where a samplesheet has FASTQ input files provided as separate entries rather than the expected single entry, an informative error should be provided. See: https://nfcore.slack.com/archives/C04CQ1LK7U1/p1734010565913889