fix(automation): persist cancelled scheduled runs

[gopi1949]

Problem

Scheduler shutdown raises asyncio.CancelledError, which bypasses except Exception. Because a TaskRun is persisted as running before execution begins, cancellation could leave that record stuck as running forever.

Fix

• Cover startup broadcast and execution with the same cancellation/finalization scope.
• Interrupt active engine/tool work, mark the existing run as error, set finished_at, persist it, and re-raise so an incomplete schedule is not advanced.
• If execution already completed and only completion notification is cancelled, preserve ok and return normally so the scheduler advances instead of replaying side effects after restart.

Tests

Regression coverage now exercises cancellation during:

• engine execution
• the startup broadcast before engine creation
• completion notification after successful execution

Six focused scheduler/automation tests pass. The full backend suite on Windows reports 1840 passed, 21 failed, and 4 skipped; the 21 failures are the existing Windows baseline and this change adds no new failures.