Add WebSocket /invocations support for realtime scenarios

[shibayan]

Priority

P3 by default; raise to P1 if realtime or voice becomes a product objective.

Goal

Support duplex Hosted Agent invocations over WebSocket for realtime and streaming workloads.

Scope

• Define connection establishment and invocation message handling.
• Support duplex streaming, cancellation, backpressure, and disconnect cleanup.
• Integrate identity and tracing with the hosted execution context.
• Add protocol tests using an in-process WebSocket client and server.

Acceptance criteria

• Multiple invocation messages can be exchanged over one connection.
• Cancellation and client disconnects stop active work.
• Backpressure has bounded memory behavior.
• Authentication and execution context match HTTP invocation behavior.
• A realtime reference sample demonstrates the supported lifecycle.

Dependency

Blocked by the HTTP Invocations protocol and its shared contracts.