[copybara-service]

[lf.agentic.Session] Keep results that come in after an action's deadline.

• While the session calls its event handler, it sets a thread-local flag
  (Session._running_event_handler()), and _log skips the deadline check
  under that flag. The flag is cleared again for the body of every action,
  including one that a handler starts. So only logs that event handlers write
  themselves, such as those of lf.agentic.SessionLogging, are exempt: a log
  from an action (session.info(...) in call) is still a cancellation
  point, as are query starts, action starts and concurrent_map, even when a
  handler makes them. The flag covers every handler and leaves the handler
  API unchanged. It is per thread, like the current action, so a handler
  running on one thread does not exempt the logs that actions write on other
  threads.
• lf.query marks a successful query completed in the else clause of its
  try. An error raised there, for example by a track_queries end
  callback, propagates as itself instead of
  AssertionError('Query is already completed.'), and the query is marked
  completed once.
• Session.track_action restores the parent action and execution in a
  finally, so they are restored even when on_action_end raises.
• Session.concurrent_map turns a TimeoutError that escapes after the
  action's deadline into ActionTimeoutError, as its docstring now says. Its
  per-item timeout is capped at the action's remaining time, so after the
  deadline such a timeout means that the action ran out of time. Before, this
  happened only as a side effect of the deadline check in handler logs
  (SessionTest.test_max_execution_time relies on it); a TimeoutError
  within the budget is still raised as is.

Behavior change: in a session with a time budget, a model reply or an action
result that comes in after the deadline is now kept, logged and seen by every
handler, and an error raised after the deadline surfaces as itself instead of
being replaced by ActionTimeoutError (except the concurrent_map timeout
above). The action's next log, query, sub-action or concurrent_map after the
deadline still raises ActionTimeoutError. In any session, an error raised in
an lf.query end callback, including a session's on_query_end handler, now
reaches the caller as itself instead of AssertionError, and the current action
is restored even when on_action_end raises.