Linter: Don't flag case operands in erb-no-unused-literals

[marcoroth]

This pull request updates the LiteralCollector in erb-no-unused-literals to skip the comparison operands of case/when and case/in statements, which were being reported as unused literals.

LiteralCollector is a blocklist-based visitor. It overrides visitCallNode to skip arguments and stops traversal into every assignment node type plus visitReturnNode/visitBreakNode/visitNextNode, but it had no handling for WhenNode, InNode, CaseNode or CaseMatchNode.

The rule bug was always there, it just wasn't reachable before #1885, PrismVisitor never descended into node list fields, so when/in branch conditions never reached the collector. Once that was fixed, every literal condition started landing in literals.

<% case value
   when "a"
     result = 1
   end %>

a.html.erb:
  2:8  ✗ Avoid using silent ERB tags for literals. `"a"` is evaluated but never used or output. (erb-no-unused-literals)

"a" isn't unused, it's the operand value is compared against. The same applied to in patterns and to a literal case subject.

Follow up on #1885
Resolves #1972

[github-actions]

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_{🌱 Grown from commit 9d1e4ab}

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commit: 9d1e4ab