fix: resume the scheduler in stop_task only if start_task paused it

[davidberenstein1957]

Extracted from #1203 (feat/add-fastapi-middleware), which bundled this with unrelated FastAPI middleware work. It changes behaviour for every start_task user, so it deserves review on its own rather than buried in a 3,700-line feature branch. #1203 will be rebased to drop the duplicated hunks.

What

start_task stops the periodic measurement scheduler so it does not interfere with the task measurement, but nothing ever restarted it. A tracker started with start() therefore lost its periodic measurements permanently after the first task.

Restarting it unconditionally in stop_task would be wrong in the other direction: users who only ever call start_task/stop_task (never start()) would be left with a 1s scheduler running that nobody asked for and nothing stops.

So start_task records whether it actually paused a running scheduler (_scheduler_paused_by_task = not self._scheduler._stopped), and stop_task resumes only in that case.

Behaviour change

• start() + start_task()/stop_task(): periodic measurement now continues after the task, as it did before the task started. Previously it stayed dead.
• start_task()/stop_task() only: unchanged — no scheduler left running.

Tests

tests/test_emissions_tracker.py::TestCarbonTracker::test_stop_task_resumes_scheduler_only_if_start_task_paused_it covers both directions. It fails on master and passes with the fix.

uv run pytest tests/ -q --ignore=tests/test_viz_data.py → 627 passed, 21 skipped. pre-commit run --all-files clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (065d0e6) to head (3609604).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1375      +/-   ##
==========================================
+ Coverage   91.39%   91.44%   +0.04%     
==========================================
  Files          49       49              
  Lines        5056     5061       +5     
==========================================
+ Hits         4621     4628       +7     
+ Misses        435      433       -2     

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