feat(middleware): define HTTP response pre-return interface

[pimlock]

Summary

Define the public HTTP_RESPONSE/PRE_RETURN contract without enabling response processing in the proxy. This lets reviewers evaluate the protobuf lifecycle and transport shape independently from the runtime implementation.

Related Issue

Part of #2691

Changes

• Add the bidirectional HttpResponsePreReturn.Evaluate service.
• Define preflight, body unit, body acknowledgement, body end, trailer, and typed session-end messages.
• Accept the HTTP_RESPONSE/PRE_RETURN manifest binding and expose response-chain description.
• Add transport-neutral in-process and remote gRPC stream interfaces.
• Add focused manifest validation coverage.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated, deferred to feat(network): inspect HTTP responses before return #3074

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated, deferred to docs(middleware): document response pre-return workflow #3075

Stack

1. feat(middleware): broaden HTTP header mutation authority #3072 — shared header mutation authority
2. feat(middleware): define HTTP response pre-return interface #3073 — response protobuf and transport interface
3. feat(network): inspect HTTP responses before return #3074 — response runtime, relay integration, and existing example
4. docs(middleware): document response pre-return workflow #3075 — documentation and middleware guide reorganization

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