Pending-prompt and elicitation tracking reworked

What

• The handling of elicitations (requests that pause a session waiting for input) was rewritten: they now register in a shared list of pending actions, can be marked as background prompts, and when a session tears down, a surviving pending action (preferring a "can use tool" request) is now republished instead of every pending request simply being rejected.
• The check used to decide whether a session is idle or still running was simplified to a single hasPendingPrompt() check (with a new promptLeftStanding guard), replacing two separate checks that used to look at pending permission requests and pending user-dialog requests independently.

Why

This makes session status (idle vs. running) and unanswered-question handling more reliable, especially for SDK and remote/cloud sessions, so a session isn't reported as idle while a question is still waiting for an answer, and pending requests aren't dropped unnecessarily when a session ends.