remote/coordinator: run reservation scheduling under lock

[alextercete]

Description
Reservation scheduling mutates shared coordinator state (reservations, places, and the derived place.reservation view), but the periodic scheduler path was performing these updates without holding the coordinator lock.

This can lead to races between the poll-based scheduler and concurrent RPC handlers (e.g. acquire/release/reservation operations), potentially resulting in inconsistent state or incorrect reservation transitions.

We now run reservation scheduling under the coordinator lock by making it async and awaiting it in the poll path. This ensures that all mutations of coordinator state are serialized consistently with the rest of the coordinator operations.

Renamed schedule_reservations to _schedule_reservations to reflect that it is now an internal async helper. All usages in the coordinator were updated. This method is not part of a public API, so renaming it is not expected to impact external users.

Testing

• Ran existing unit tests covering reservation and acquire/release flows.
• Exercised reservation creation/cancel and place acquire/release scenarios manually.
• No deadlocks were observed in these runs.
• This patch does not add a dedicated stress test, and the current tests do not prove the absence of deadlocks.

Performance

• Measured _schedule_reservations runtime under load. With 100 places and 200 reservations created in parallel, observed runtime was ~0.0013s, with worst case ~0.0036 s.
• RPC paths were already invoking reservation scheduling while holding the coordinator lock; the primary change is that the poll-based scheduler path is now also serialized under the same lock.

This patch focuses on fixing the scheduler locking model and does not change coordinator scheduling behavior.

Checklist

• PR has been tested

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.0%. Comparing base (06fd06c) to head (4cc7ac6).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/coordinator.py	0.0%	4 Missing ⚠️

Additional details and impacted files

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- Coverage    46.0%   46.0%   -0.1%     
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  Files         180     180             
  Lines       14462   14464      +2     
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  Hits         6654    6654             
- Misses       7808    7810      +2     

Flag	Coverage Δ
3.10	46.0% <0.0%> (-0.1%)	⬇️
3.11	46.0% <0.0%> (-0.1%)	⬇️
3.12	46.0% <0.0%> (-0.1%)	⬇️
3.13	45.9% <0.0%> (-0.1%)	⬇️
3.14	45.9% <0.0%> (-0.1%)	⬇️

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[Emantor]

I agree with the locking, however I do not see what we gain from turning _schedule_reservation() into an async function.