fix: Preserve count=0 in RunClient's charge (#751)

## Summary

`RunClient.charge` (sync + async) built the request body with `'count':
count or 1`. Because `or` treats `0` as falsy, a caller passing
`count=0` — e.g. `count=len(batch)` for an empty batch, or a defensive
no-op — was silently sent as `count=1`. On a pay-per-event billing
endpoint that's a real overcharge, not a cosmetic bug.

## Fix

Changed the signature from `count: int | None = None` to `count: int =
1` and pass `count` through to the request body unchanged. `0` now
reaches the server as `0`; the default `1` is expressed in the signature
instead of via a fallback.

Docstring updated to reflect the new contract.

## Tests

`tests/unit/test_run_charge.py` parametrizes the sync and async clients
over `count` values `0`, `1`, `5` and asserts the value lands in the
request body verbatim. The `0` case fails on `master` and passes here.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>