Fix scaling plot validation

[linsword13]

Previously the check was performed on a string, such that it gives false positive if the FOM name is a single character.

[gemini-code-assist]

Code Review

This pull request refactors the validate_spec method in reports.py to correctly validate the passed chart_spec argument instead of iterating over self.spec. It also updates the corresponding tests and adds a new unit test, test_scaling_plot_invalid_spec, to ensure that scaling plot specifications with fewer than two arguments are properly rejected. There are no review comments, so I have no feedback to provide.

[ramble-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: f6f4704

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.0849	2.1005 (de4ccce)	2.0984
test_large_template_expansion	passed	1.5823	1.6327 (de4ccce)	1.6532
test_many_experiments	passed	24.1183	24.6595 (de4ccce)	25.4157
test_many_objects_defaults	passed	13.7158	14.4773 (de4ccce)	14.4945
test_matrix_filter_perf	passed	1.1483	1.1632 (de4ccce)	1.1704

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (de4ccce) to head (f6f4704).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1718   +/-   ##
========================================
  Coverage    93.48%   93.48%           
========================================
  Files          367      367           
  Lines        35916    35926   +10     
========================================
+ Hits         33575    33586   +11     
+ Misses        2341     2340    -1     

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