Migrate endpoint feature flags to OpenFeature

[coderabbitai]

Summary

Replace the custom runtime endpoint feature-flag implementation with an OpenFeature-based implementation.

Rationale

Pull request #286 introduces local environment-variable feature flags and router registration helpers. OpenFeature can provide a standard feature-flag API and allow future provider integration without changing endpoint registration logic.

Affected areas

• src/trustyai_service/service/config/feature_flags.py
• src/trustyai_service/service/config/registry.py
• src/trustyai_service/main.py
• Feature-flag and application integration tests under tests/service/config/ and tests/test_app_integration.py

Required changes

• Evaluate OpenFeature support for the Python service and select a supported provider strategy.
• Replace direct reads from the current ENDPOINTS mapping with OpenFeature flag evaluation.
• Preserve the existing group-level and per-metric endpoint gating behavior.
• Preserve documented environment-based configuration where required, or document the migration path.
• Update unit and integration tests for the OpenFeature implementation.
• Remove the superseded custom feature-flag code after the migration.

Acceptance criteria

• Endpoint registration uses OpenFeature evaluation.
• Fairness, drift, and explainer endpoint gating retains the current intended behavior.
• Tests cover default values, configuration overrides or provider values, group-level gating, and per-metric gating.
• The custom feature-flag implementation is removed or reduced to a documented compatibility layer.

Backlinks

• Originating PR: feat(config): add feature flags for runtime endpoint gating [RHOAIENG-55917] #286
• Requesting comment: feat(config): add feature flags for runtime endpoint gating [RHOAIENG-55917] #286 (comment)
• Requested by: @ruivieira