fix(acp-http-client): isolate detached POST failures

[RGHenderson]

Summary

• reject only the matching JSON-RPC request when its detached POST fails
• preserve the shared SSE transport so later requests can continue
• add a real-server integration test covering failure followed by a successful prompt

Motivation

postMessage runs detached so long-running requests do not block client replies. A network failure in one detached POST currently errors the shared readable stream, terminating an otherwise healthy SSE connection and every pending or subsequent request.

Rationale

Requests already have JSON-RPC IDs, so a POST failure can be represented as an error response for that request. Notifications have no response to reject and retain the existing transport-fatal behavior.

The broader proposal to return 202 Accepted for long-running requests and deliver completion over SSE is tracked separately in #305.

Test plan

• pnpm --filter acp-http-client typecheck
• build sandbox-agent and run pnpm --filter acp-http-client test
• verify a failed prompt POST rejects that prompt and a second prompt succeeds on the same connection

Fixes #307