refactor(epp): unexport RequestContext protocol and response state

[LukeAVanDrie]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Issue #1189 proposed splitting RequestContext into a protocol struct and a request-state struct because internal layers (director, admission control) could reach raw Envoy ext_proc machinery. Most of that machinery is already unexported: the gRPC response buffers, the streaming flag, and the processing-duration bookkeeping are package-private on main. Unexporting the remaining package-private surface enforces the boundary at compile time and makes the struct split unnecessary.

Unexported:

• The ext_proc state machine: the RequestState field, the StreamRequestState type and its constants, and the RequestDroppedReason field.
• Response bookkeeping with no users outside the handlers package: FirstTokenTimestamp, LastChunkReceivedTimestamp, ResponseCompleteTimestamp, ResponseSize, ResponseComplete, ResponseStatusCode, RequestRunning.

Deleted dead protocol code: the reqTrailerResp field was never assigned, so its send branch in updateStateAndSendIfNeeded could not execute, and the TrailerRequestResponsesComplete and TrailerResponseResponsesComplete constants were never referenced. The state constants are package-private and their values never leave the process, so the hand-numbered values are replaced with iota.

The struct fields are grouped by owner (request-control layers vs the handlers package), and the TODO referencing kubernetes-sigs/gateway-api-inference-extension#2082 (closed unimplemented) is replaced with a doc comment stating the visibility rule.

Exported fields with users outside the package are unchanged: Request, Response, SchedulingRequest, TargetPod, TargetEndpoint, TargetEndpointScores, the model names, ObjectiveKey, Priority, RequestReceivedTimestamp, RequestSize, Usage, ResponseBodyStarted, Parser. Out-of-tree implementations of the Director interface lose compile-time access to the newly unexported fields; no in-tree consumer reads them, and they carry ext_proc protocol state with no external use.

Observed while auditing field usage, left out of scope:

• Priority duplicates SchedulingRequest.Objectives.Priority. Deriving it would change the priority label on error metrics for requests that fail between the two assignments.
• ResponseBodyStarted is director-maintained state on the shared struct. Relocating it belongs with future director-state work.

Which issue(s) this PR fixes:

Fixes #1189

Release note (write NONE if no user-facing change):

NONE

[ahg-g]

/lgtm
looks good, pls rebase.

[LukeAVanDrie]

/lgtm looks good, pls rebase.

thanks did a merge commit, though the conflict was only a single line from another one of my PRs that merged earlier

[elevran]

@ahg-g approved