Refactor: Split `RequestContext` into `ProtocolContext` and `RequestState`

[LukeAVanDrie]

What would you like to be added:

Consider refactoring pkg/epp/handlers/RequestContext. Currently, it is a monolithic struct containing:

• Envoy External Proc protocol fields (reqHeaderResp, respBodyResp)
• Request metadata (IncomingModelName)
• Lifecycle state (RequestRunning, ResponseComplete, TargetPod)

Why is this needed:

As noted in the code TODO, this object exposes too much data to too many layers. As we add complex features, the lack of isolation makes it difficult to reason about the request lifecycle state machine.

We should split this into:

1. ProtocolContext: Handles the gRPC/Envoy interaction.
2. RequestState: Tracks the logical request lifecycle (Start/End times, Pod assignment).