fix: preserve error context in validation error handling

[DeVikingMark]

Problem

Validation functions were discarding error context using Err(_) pattern,
making debugging difficult when protobuf decode or signature verification fails.

Solution

• Added new ValidationError variants to preserve error context
• Updated error handling in message, verification, and cast validation
• Aligns with existing error handling patterns in HubError and TrieError

[vercel]

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