Add test documenting variable binding persistence in failed patterns

[Copilot]

Addresses clarification from @hasumikin that variables bound during pattern matching persist even when the pattern fails to match completely, matching CRuby semantics.

Changes

• Added test case documenting variable binding behavior across failed pattern clauses

Example

a = 99
result = case [1, 2]
in [a, 3]      # Fails to match, but a is bound to 1
  :first
in [b, 2]      # Matches, a still has value 1 (not 99)
  [a, b]
end
p result       #=> [1, 1]

This confirms the current implementation in PM_LOCAL_VARIABLE_TARGET_NODE (lines 687-696 of src/codegen_prism.inc) is correct and matches CRuby behavior.

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