Improve error handling and panic messages in Rust evaluator

[arnaudgelas]

This commit addresses multiple panic sites in the Rust evaluator to improve debuggability and provide safer alternatives for error handling.

value.rs - New safe accessor methods:

• Add try_as_int(), try_as_bool(), try_as_str() returning Result<T, QuintError>
• Add try_as_map(), try_as_list(), try_as_record_map(), try_as_variant()
• Add type_name() helper for generating informative error messages
• These provide safe alternatives for callers that need to handle type mismatches gracefully rather than panicking

value.rs - Improved panic messages:

• All as_* methods now include the actual type name in panic messages (e.g., "Expected integer, got boolean" instead of "Expected integer")
• cardinality() and contains() panics now show type names
• Lambda hash/comparison panics now explain that lambdas are not hashable in Quint

builtins.rs - Convert panics to runtime errors:

• Unknown lazy/eager operators now return QNT500 error instead of panicking
• This allows better error recovery and more informative error messages
• getOnlyElement unwrap replaced with documented expect

The existing as_* methods are retained for internal use where type correctness is guaranteed by the type checker, but now provide better debugging information if assumptions are violated.

• 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