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 80.26%. Comparing base (284af39) to head (451569b).
⚠️ Report is 1 commits behind head on development.

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          80.26%   80.26%   -0.01%     
- Complexity          4460     4481      +21     
=================================================
  Files                293      293              
  Lines              16381    16424      +43     
=================================================
+ Hits               13149    13182      +33     
- Misses              3232     3242      +10     

Flag	Coverage Δ
unittests	80.26% <75.92%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.