feat(subscribe): support gRPC server-streaming subscriptions

[jolestar]

Scope

Support gRPC server-streaming methods through uxc subscribe.

Why

This is useful, but it is a separate track from the WebSocket-based subscription work. The current gRPC adapter still rejects non-unary calls.

Deliverables

• Support server-streaming gRPC methods in the subscription runtime
• Preserve auth/header handling where supported by the current gRPC invocation path
• Add local E2E coverage for gRPC server streaming -> file sink

Non-goals

• No client-streaming or bidi-streaming support in the first iteration
• No attempt to merge this with the WebSocket runtime issues