Ignore non-boolean Rust test results

[puneetdixit200]

Fixes #1941

Summary

• Treat non-boolean Rust test results as ignored instead of panicking.
• Add a safe boolean accessor for evaluator values.
• Add regression coverage for a run that returns a non-boolean value.

Tests

• PATH="$PWD/.test-bin:$PATH" CARGO_HOME=$PWD/evaluator/.cargo-home CARGO_TARGET_DIR=$PWD/evaluator/target cargo test --manifest-path evaluator/Cargo.toml --test tester_tests --locked
• PATH="$PWD/.test-bin:$PATH" CARGO_HOME=$PWD/evaluator/.cargo-home CARGO_TARGET_DIR=$PWD/evaluator/target cargo test --manifest-path evaluator/Cargo.toml --locked
• QUINT_HOME="$PWD/.quint-home" PATH="$PWD/.test-bin:$PATH" quint test examples/language-features/counters.qnt --backend=rust
• git diff --check

AI assistance was used: OpenAI GPT-5 helped draft and review the implementation. I reviewed, tested, and take responsibility for the final changes.

• I have read and I understand the Note on AI-assisted contributions
• Changes manually tested locally and confirmed to work as described
  (including screenshots is helpful)
• Tests added for any new code
• Documentation added for any new functionality
• Entries added to the respective CHANGELOG.md for any new functionality

Documentation and changelog entries are not applicable because this is a bug fix, not new functionality.