fix: preserve count=0 in RunClient.charge

[vdusek]

Summary

RunClient.charge (and its async counterpart) built the request body with 'count': count or 1. The short-circuit or treats 0 as falsy, so a caller passing count=0 — e.g. count=len(batch) where the batch ended up empty, or a defensive no-op — was silently billed as count=1. On a pay-per-event billing endpoint the difference is financially material.

Replaced count or 1 with an explicit count if count is not None else 1 at both call sites, and clarified the docstring so the count=0 behaviour is documented rather than implicit.

Added a parametrized unit test (tests/unit/test_run_charge.py) covering None, 0, 1, and 5 for both sync and async clients; the 0 case fails on master and passes with this fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.22%. Comparing base (9ab096a) to head (0faf061).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #751   +/-   ##
=======================================
  Coverage   95.22%   95.22%           
=======================================
  Files          45       45           
  Lines        5154     5154           
=======================================
  Hits         4908     4908           
  Misses        246      246           

Flag	Coverage Δ
integration	95.22% <ø> (ø)

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