Validate present optional record fields at runtime

[OskarEichler]

Summary

Runtime record validation checks required fields but ignores optional_fields entirely. A present optional field therefore passes even when its value is incompatible with the declared type.

Validate optional fields only when their key is present, preserving their optional presence contract.

Verification

• baseline model accepts an invalid present optional value; focused and cumulative candidates reject it while accepting absent/valid values
• runtime type-checking tests: 15 tests / 302 assertions
• 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.