fix(aggregations): guard mean_of_measurement against empty measurements

[WatchTree-19]

what

mean_of_measurement divides by len(measurements) with no empty guard:

measurements = get_measurements(measurement_name, items)
total = sum(measurements)
return total / len(measurements)

so a test whose aggregate_measurements() receives no measured items - e.g. every item errored or was filtered out - raises ZeroDivisionError instead of producing a score, crashing aggregation/reporting.

its sibling in the same module, MeasurementStats.calculate, already guards this and returns mean 0 for an empty input. mean_of_measurement now does the same, keeping the two consistent.

fix

return 0.0 when there are no measurements. adds regression tests (test_mean_of_measurement and test_mean_of_measurement_no_measurements) - the empty case raises ZeroDivisionError on main and passes with the change. black clean, DCO signed.

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[sylvesterkaczmarek]

Could an empty set remain distinguishable from a measured zero? Returning 0.0 means an all-error or all-filtered run becomes a valid zero measurement. For safety benchmarks that can make missing coverage look benign and bias aggregates. I would prefer NaN / None, or an explicit no-data result, rather than converting absence of evidence into a score.