feat: add getDanfse for ADN DANFSE artifact retrieval

[vitormattos]

Summary

Adds support for retrieving DANFSE (PDF) artifacts from the ADN (Ambiente de Dados Nacional) endpoint.

Changes

• ArtifactException: new typed exception for DANFSE retrieval failures
• NfseErrorCode::ArtifactRetrievalFailed: new error code enum case
• EnvironmentConfig::danfseBaseUrl: auto-configured URL for ADN DANFSE endpoint (sandbox/production)
• NfseClientInterface::getDanfse: new contract method
• NfseClient::getDanfse + getRawBytes: implementation using plain HTTPS (no mTLS), returns binary PDF bytes
• Tests covering all new code paths

ADN DANFSE endpoint

• Production: https://adn.nfse.gov.br/danfse/{chaveAcesso}
• Sandbox: https://adn.producaorestrita.nfse.gov.br/danfse/{chaveAcesso}