feat(acp): implement session/close handler and capability advertisement

[bug-ops]

Context

ACP schema 0.11.2 renamed session/stop → session/close (breaking). Zeph has no explicit session/close handler in zeph-acp — routing currently falls through to the SDK crate.

More critically, Zeph must:

1. Handle the session/close method in the ACP router (terminate the session, cancel in-flight turns, clean up AcpSessionManager state)
2. Advertise session: { close: {} } in the capabilities response (initialize → serverInfo.capabilities) — without this advertisement Zed will not know the capability is present
3. Ensure AcpSessionManager drops the session and cancels any in-flight turn on receipt

Why it matters

Zed IDE calls session/close on thread close (e.g., when the user closes a conversation tab). Without a handler, sessions silently leak in AcpSessionManager. The session/close capability advertisement is also required for ACP Registry inclusion (registry CI checks capabilities).

Acceptance criteria

• session/close JSON-RPC method handled in zeph-acp/src/handler.rs (or equivalent router)
• session: { close: {} } present in initialize response capabilities object
• AcpSessionManager removes the session from the LRU and sends cancel signal to any running turn actor
• Schema 0.11.2 crate update (covered by feat(acp): update agent-client-protocol 0.10.2→0.10.3 + schema 0.11.2→0.11.4 — logout, elicitation, NES, stabilized session/list #2411) is prerequisite

Related

• feat(acp): update agent-client-protocol 0.10.2→0.10.3 + schema 0.11.2→0.11.4 — logout, elicitation, NES, stabilized session/list #2411 — ACP crate update + elicitation + logout + session/list stabilization
• fix(acp): discovery endpoint hardcodes "protocol_version": "0.9" — should reflect actual ProtocolVersion::LATEST #2412 — discovery manifest protocol_version type fix