fix: lock CPU power history swap

[davidberenstein1957]

Fixes a lost-update race on CPU._power_history.

What changed

CPU._power_history is appended to by the 1 Hz monitor scheduler thread (CPU.monitor_power) and drained by the measure_power_secs scheduler thread (CPU.total_power) — two distinct PeriodicScheduler timer threads (codecarbon/emissions_tracker.py:345-352). The read-then-rebind in total_power was not atomic, so any sample appended between the list comprehension and self._power_history = [] went into the discarded list.

• Added a threading.Lock to CPU, held only for the O(1) list swap. _get_power_from_cpus() is called outside the lock in both paths, so a slow backend (e.g. the IntelPowerGadget subprocess) never blocks the monitor thread.
• Removed the unreachable if not power_history_in_W: branch — a sample is unconditionally appended before the average, so the list is never empty.

Behaviour is otherwise unchanged, including the extra synchronous sample taken at measurement time.

Why

In cpu_load mode the reported cpu_power is the mean of the buffered samples. The samples dropped in the swap window are exactly those taken while a slow measurement is in flight, so the resulting error is small but systematic and silent.

How it was verified

New test tests/test_cpu_load.py::TestCPULoad::test_cpu_total_power_keeps_samples_added_while_draining makes the race deterministic (a list subclass that fires monitor_power() at iteration exhaustion, i.e. inside the lost-update window) and asserts the injected sample survives. It fails on master and passes with this change. uv run pytest tests/test_cpu_load.py -q → 9 passed.

Note: uv run task format reformats a large number of unrelated files on the current tree, so only the two touched files are included here.

Closes #1315

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.48%. Comparing base (065d0e6) to head (d70b9ae).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1335      +/-   ##
==========================================
+ Coverage   91.39%   91.48%   +0.08%     
==========================================
  Files          49       49              
  Lines        5056     5059       +3     
==========================================
+ Hits         4621     4628       +7     
+ Misses        435      431       -4     

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