fix(merqury): make optional spectra fl and st png outputs

[MohineTAlam]

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Closes #XXX

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Description:
Mark spectra-cn and spectra-asm fl/st PNG outputs as optional.

These plot variants are not consistently produced by Merqury across datasets,
while ln (log-scale) plots are reliably generated and retained as required
outputs to ensure QC completeness.

This prevents workflow failures due to missing non-essential plot files.

(linting and testing done)

[LouisLeNezet]

Could you add (optional) in the meta.yml for each of the optional output ?
If possible a nf-test that don't produce these files would be nice.
Good to go otherwise.

[MohineTAlam]

Thanks! I've added optional: true to the corresponding outputs in meta.yml to keep it consistent with the module outputs.

These plot variants are not consistently produced by Merqury across datasets, so marking them optional reflects the actual behaviour.

[MohineTAlam]

Hi let me know if there are any issues with this change :)

[LouisLeNezet]

Looks good to me !