Refactor Micronaut HttpClient from blocking to reactive (via build file)

[Jonathan-Zollinger]

Summary

Refactor the current Micronaut HttpClient usage from a blocking client to a reactive client. This change should be implemented by updating the client dependency/configuration via the build file (e.g., Gradle), rather than ad-hoc code changes scattered across modules.

Acceptance Criteria

• Update the build file to use/configure the reactive client for the project (Gradle change is the source of truth).
• Update application wiring / call sites as needed to use reactive return types (no blocking wrappers).
• Update ALL existing tests (unit/integration) to reflect reactive behavior and ensure they pass.
• Ensure CI passes (test suite + any relevant checks).