Fix: TryFrom for FieldValue conversions

Fixes #177. As I've stated in the issue, I'm pretty sure this is a non-breaking change. <!-- This is an auto-generated comment: release notes by coderabbit.ai --> ## Summary by CodeRabbit * **Refactor** * Standardized conversion implementations for FieldValue variants to a single, consistent direction and receiver style. * Consolidated owned and referenced conversion paths to reduce duplication while preserving user-facing error messages. * No behavioral changes; public usage remains compatible aside from the standardized conversion signatures. <!-- end of auto-generated comment: release notes by coderabbit.ai -->