Add require_categories DSL and fix ActiveSupport dependency

[DmitrySychev]

Summary

• Add require_categories true DSL macro for multi-label classifiers that should fail when no categories match
• Fix .blank? ActiveSupport dependency in Web content fetcher
• Bump to 0.2.0

Details

require_categories

Multi-label classifiers previously always returned Result.success even with an empty categories array. This made it impossible to distinguish "the LLM found no matching categories" from "classification succeeded with results."

class MyClassifier < LlmClassifier::Classifier
  categories :a, :b, :c
  multi_label true
  require_categories true  # now returns Result.failure when categories is empty
end

.blank? fix

ContentFetchers::Web#normalize_redirect_url called .blank? which is an ActiveSupport method. Replaced with nil? || empty? so the gem works without Rails/ActiveSupport.

[DmitrySychev]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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