feat: add VCTM schema validation before credential issuance

[leifj]

• Add validation package (pkg/sdjwtvc/validation.go) with comprehensive document validation against VCTM schema

• Validate mandatory claims presence and structure

• Support nested claim paths and array values

• Provide detailed field-level error messages

• Auto-allow standard JWT claims (iss, exp, vct, cnf, etc.)

• Support strict/non-strict validation modes

• Integrate validation into MakeSDJWT() handler before BuildCredential()

• Prevents invalid credentials from being signed

• Returns clear error messages to API clients

• Add comprehensive test coverage:

  • 13 validation unit tests (success/error cases, nested claims, strict mode)
  • 4 integration tests verifying end-to-end validation

• Update test infrastructure:

  • Add mandatory family_name claim to test VCTM
  • Update mock test data for EHIC, PID, and Diploma credentials

All tests passing (12 test suites, 0.148s execution time).