Function session_end_idempotent 

pub fn session_end_idempotent(
    endpoint: &str,
    session_id: &str,
) -> Result<Option<SessionStats>, IpcError>

End a session, treating a vanished daemon as success.

This is the shared library entry point for ending a session. It is the contract used by the CLI’s zccache session-end <uuid> subcommand AND by any in-process caller (e.g. soldr’s at-exit rust-plan save) — both must agree on what “the daemon already died” means.

Return shape

• Ok(Some(stats)) — daemon was reached and returned stats for the session.
• Ok(None) — daemon was reached but returned no stats (session was tracked without stats), OR the daemon was unreachable at connect time. Both are no-ops from the caller’s perspective: the session is implicitly ended when the daemon dies (see #137 for the daemon-side mirror), and a caller that just wants to “end the session, don’t care if the daemon is still alive” should treat both as success.
• Err(IpcError) — anything else: timeouts, protocol mismatches, send/recv mid-conversation failures, daemon error responses. These are real faults and must be surfaced.

Why a separate function

Issue #159: soldr was failing Windows CI on every main commit because its in-process session-end (called from rust-plan save) did not share code with cmd_session_end, so #151’s connect-failure idempotency only applied to the CLI subprocess path. Promoting this contract to the library lets all callers — current and future — share the same behavior.