fix: treat naive datetime query params as UTC (#752)

## Summary

`HttpClientBase._parse_params` called `.astimezone(UTC)` directly on
incoming `datetime` values. Per Python's stdlib, a naive datetime is
presumed to represent time in the system's local timezone, so on a
non-UTC host the value was shifted by the host's offset and then
serialized with a `Z` suffix — a wire format that *looks* like UTC but
isn't. This silently corrupted timestamp filters such as
`started_before`/`started_after` on `runs().list()` whenever a caller
passed a naive datetime.

The fix attaches `UTC` to the value when `tzinfo` is missing, so naive
datetimes round-trip as UTC instead of being relocalized.

Added a unit test that pins `TZ=Asia/Karachi` with `time.tzset()` to
reproduce the bug deterministically on Unix CI (skipped on Windows,
where `time.tzset` is unavailable).

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