Restore scoped non-simple method type assertions - #3139

Summary

allow_non_simple_method_type resets its flag only through rescue. It therefore swallows exceptions raised by the caller, fails to restore state after a successful scope, and breaks nested scopes by resetting the outer allowance.

Capture the prior value and restore it in ensure, allowing exceptions to propagate normally.

Verification

• baseline models reproduce exception swallowing, successful-state leakage, and nested-scope loss; focused and cumulative candidates restore all three contracts
• unit-test assertion coverage is included in the cumulative focused models
• all 109 runtime files compile on Ruby 4.0.6
• focused candidate syntax, RuboCop, and diff checks pass

Source-only change; no test file is included.