MX-225: Self-Service API Fields Filtering Defect Fix

[DeathGun44]

Overview

Fixes the defect matching MX-225 where Self-Service APIs return stripped, empty JSON objects (returning {}) for nested collections when filtering queries using the ?fields= parameter.

Root Cause

When omitting the list of supported parameters (_DATA_PARAMETERS) during serialization, Gson defaults to an aggressive InclusionStrategy which improperly strips all inner properties of child arrays unless explicitly requested.

Changes Made

• API Fix: explicitly injected ClientApiConstants.CLIENT_ACCOUNTS_DATA_PARAMETERS into SelfClientsApiResource#retrieveAssociatedAccounts during the serialization step. This securely enforces an ExclusionStrategy preserving internal property states.
• Unit Testing: Included Mockito assertions verifying correct strategy enforcement.
• Integration Validation: Added a rigorous Testcontainer-backed E2E test (SelfClientsApiIntegrationTest) orchestrating live Savings Applications to statically guarantee inclusive & exclusive REST JSON filtering outputs precisely match core Fineract behavior.

[IOhacker]

Lgtm