fix(nmr): handle nested arrays when extracting spectra metadata

[vcnainala]

Summary

• Stop nmrxiv:extract-dataset-spectra-info from failing with Array to string conversion on NMRium payloads that store 2D frequencies, filters, meta, or probe fields as arrays.
• Flatten nested scalars for search text and take the first usable value for dedicated columns (nucleus, frequency, probe name, etc.).
• Cover the nested-array case in DatasetSpectraInfoExtractorTest.

Test plan

• php artisan test tests/Unit/Support/DatasetSpectraInfoExtractorTest.php
• Re-run php artisan nmrxiv:extract-dataset-spectra-info and confirm previously failing dataset IDs extract without errors
• Rebuild stats with php artisan nmrxiv:index-spectra-metadata-stats

[codecov]

Codecov Report

❌ Patch coverage is 75.92593% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.32%. Comparing base (7480827) to head (222c902).

Files with missing lines	Patch %	Lines
app/Support/Nmr/DatasetSpectraInfoExtractor.php	75.92%	13 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             development    #1530      +/-   ##
=================================================
- Coverage          80.34%   80.32%   -0.03%     
- Complexity          4416     4437      +21     
=================================================
  Files                289      289              
  Lines              16241    16284      +43     
=================================================
+ Hits               13049    13080      +31     
- Misses              3192     3204      +12     

Flag	Coverage Δ
unittests	80.32% <75.92%> (-0.03%)	⬇️

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