feat: emit flat structured logging contract fields

[ramioooz]

Parent: digital-bank-java/.github#91

Summary

• Emit the required flat structured fields: timestamp, level, service, environment, correlation_id, trace_id, logger, and message.
• Preserve Spring Boot ECS output additively, including ECS service and tracing members and existing redaction behavior.
• Use Spring Boot structured JSON rename and customizer support; no collector-only projection and no transaction-service changes.
• SIT-only rollout; no AWS, UAT, or PROD changes.

Verification

Validation: ./mvnw --batch-mode --no-transfer-progress verify; BUILD SUCCESS; 57 tests.

SIT verification

• Config Server was temporarily pointed at this branch, rendered application-sit.yml, and returned logging.structured.format.console=ecs; the live Config Server and customer deployment were restored afterward.
• A restarted SIT customer pod emitted the flat fields and preserved ECS fields in application JSON, including timestamp, level, service, environment, correlation_id, trace_id, logger, message, ecs_service, nested log, and ecs.version.
• OpenSearch currently receives this Docker Desktop path as an envelope with the application JSON under structured.log, so field-capability queries do not expose the inner service fields yet. This is the existing Fluent Bit and infra ingestion limitation; this change deliberately keeps the service-emitted contract authoritative rather than replacing it with collector projection.