[Bug]: Policy Engine checks Mode() and Implements relevant interfaces in Header modes

[renuka-fernando]

Please select the area the issue is related to

Area/Gateway (Routing, API deployment in gateway etc.)

Please select the aspect the issue is related to

Aspect/API (API backends, definitions, contracts, interfaces, OpenAPI)

Description

Fix the inconsistency between streaming mode, buffered mode, and the header.

1. Check Mode()
2. If mode is defined, check that the interface is implemented.

[renuka-fernando]

@sehan-dissanayake