feat(acp): complete prompts asynchronously over SSE

[RGHenderson]

Summary

• return 202 Accepted once session/prompt has been written to the agent
• deliver the eventual JSON-RPC result, timeout, or process-exit error over the existing SSE stream
• preserve synchronous handling for short requests and immediate HTTP errors before acceptance
• document and test the updated transport contract

Motivation

Long-running prompt POSTs currently remain open for the entire agent turn. Their success therefore depends on every client and intermediary allowing sufficiently long response-header timeouts, even though the client already maintains an SSE stream capable of carrying correlated JSON-RPC responses.

Rationale

session/prompt is the long-running operation that needs asynchronous HTTP acceptance. Registering its request ID before writing to the agent preserves response correlation and ring-buffer replay. Returning 202 only after the stdin write succeeds keeps validation and write failures synchronous, while detached timeout and process-exit handling ensure every accepted request still receives a terminal JSON-RPC envelope.

Other requests keep their existing 200 response-body behavior, and notifications retain their existing 202 behavior. This PR is independent of the request-scoped client reliability fix in #306.

Closes #305.

Test plan

• cargo fmt --all -- --check
• cargo check -p acp-http-adapter -p sandbox-agent
• cargo test -p acp-http-adapter
• pnpm --filter acp-http-client typecheck
• rebuilt sandbox-agent and ran pnpm --filter acp-http-client test
• Docker-backed acp_round_trip_and_replay in CI (the local in-process image build stalled before creating a container)

[RGHenderson]

@NathanFlurry WDYT? Left some open questions in the attached issue.