Add shared author_creditable tests for all AuthorCreditable models

[maebeale]

What is the goal of this PR and why is this important?

• The AuthorCreditable concern (#author_credit) formats author names based on preference (full name, first name + last initial, first name only, last name only, anonymous)
• Only 2 of 6 models using this concern had tests, and those were missing coverage for first_name_last_initial and last_name_only preferences
• This PR ensures all name display preferences are tested across every model that uses the concern

How did you approach the change?

• Created a shared example (spec/support/shared_examples/author_creditable.rb) following the existing featureable and mentioner shared example patterns
• Tests all 7 scenarios: full_name, first_name_last_initial, first_name_only, last_name_only, anonymous, nil fallback, and no-person fallback
• Automatically skips the nil fallback test for models that validate presence of author_credit_preference (StoryIdea, WorkshopVariationIdea)
• Replaced inline tests in Story and Workshop specs with the shared example
• Added shared example to StoryIdea, WorkshopIdea, WorkshopVariation specs
• Created new spec file for WorkshopVariationIdea

Anything else to add?

• 141 examples, 0 failures across all 7 affected spec files
• Net effect: -71 lines of duplicated tests, +69 lines of shared + comprehensive coverage

🤖 Generated with Claude Code