Improve error path test coverage

[Folyd]

Audit and improve test coverage of error handling code paths throughout the SDK.

Tasks

• Audit all modules for error paths that lack tests
• Add negative test cases for:
  • Invalid VID formats
  • Cryptographic operation failures
  • Malformed CESR messages
  • Network/transport errors
  • Invalid message signatures
  • Expired or revoked credentials
• Test boundary conditions (empty inputs, maximum sizes, etc.)
• Add tests for concurrent error scenarios
• Document common error testing patterns

Acceptance Criteria

• All error enum variants have at least one test
• Coverage of error-returning functions is >90%
• Edge cases and boundary conditions are tested
• Documentation includes error testing examples

Modules Needing Attention

• cesr/ - Decoder error paths
• crypto/ - Signature/encryption failures
• vid/ - VID verification failures
• transport/ - Network error handling
• store.rs - Database/storage errors

Priority

High - Critical for security and reliability

Part of #245