Fix navigation cleanup under all exit paths

[pascalzauberzeug]

Motivation

With rosys#403, the automator now closes the wrapped coroutine on stop, raising GeneratorExit inside start(). Awaiting self.finish() from the finally block was not safe under.

Implementation

• Replace the _is_prepared boolean with _finish_task: Task | None that tracks the actual in-flight cleanup.
• New is_ready property combines "no upcoming path" and "no cleanup in flight"; start() early-returns with a user-visible warning if called while a previous run is still cleaning up.
• In the finally of start(), schedule finish() via rosys.background_tasks.create(...) instead of awaiting it — this sidesteps the GeneratorExit constraint while keeping cleanup tied to the automation's lifecycle.
• finish() no longer guards on _is_prepared; it clears _finish_task at the end and notifies the user that the navigation was cleaned up.
• Bump the pinned rosys revision to include rosys#403.

Risks / Notes

• finish() now runs detached from start(), so wheels stop and implement deactivation complete shortly after the automator reports done. Subscribers to AUTOMATION_STOPPED / AUTOMATION_FAILED that assumed cleanup had finished must be reviewed.
• If finish() itself raises, _finish_task is never cleared and the navigation stays in a non-ready state until the process restarts.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• I chose meaningful labels (if GitHub allows me to so).
• I documented breaking changes and set the 'breaking change' label if needed
• The implementation is complete.
• Tests with a real hardware have been successful (or are not necessary).
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).