HCW-345 add sandbox-only lambda handler

[jolyonbrownnhs]

Summary

• route sandbox deployments to a dedicated Lambda handler instead of branching inside the normal handler path
• serve Practitioner and PractitionerRole sandbox responses from packaged spec example JSON files
• remove the legacy SANDBOX_MODE/mock LDAP fallback and keep only status endpoint pass-through for sandbox

Testing

• poetry run pytest src/request_handlers/sandbox_responses_test.py src/ldap/connection_fetch_test.py src/main_test.py src/request_handlers/practitioner_test.py src/request_handlers/practitioner_role_test.py