feature: add SLIMRPC, WebSocket, and stdio transport bindings

[muscariello]

Type: Feature

Summary

Add first-class SLIMRPC, WebSocket, and stdio transport bindings across the A2A Rust workspace.

Why

The workspace currently supports JSON-RPC over HTTP, REST / HTTP+JSON, gRPC, and SSE streaming. This feature tracks the next transport expansion across the existing transport seams: protocol constants in the core crate, client transport registration in the factory, transport-agnostic server wrappers around RequestHandler, and end-to-end transport coverage in the helloworld example.

Tasks

• task: add SLIMRPC transport binding #13 task: add SLIMRPC transport binding
• task: add WebSocket transport binding #14 task: add WebSocket transport binding
• task: add stdio transport binding #15 task: add stdio transport binding

Scope / plan

• Extend protocol inventory and negotiation surfaces for SLIMRPC, WebSocket, and stdio
• Add client transport implementations and registration paths for all three bindings
• Add corresponding server wrappers or runtime helpers where the binding is server-facing or subprocess-backed
• Define consistent ServiceParams propagation, streaming semantics, and A2AError mapping across the three bindings
• Extend end-to-end transport coverage and docs for each binding

Acceptance criteria

• All three bindings have explicit protocol identifiers and documented negotiation behavior
• Client negotiation can select the new bindings alongside existing JSON-RPC, REST, and gRPC support
• Server-facing bindings bridge into RequestHandler without changing executor or task-store semantics
• End-to-end coverage demonstrates parity for key unary, streaming, and error paths