Fix timezone-aware datetime conversion

[hownowstephen]

Summary

• _datetime_to_timestamp() used dt.replace(tzinfo=timezone.utc) which silently discards existing timezone info instead of converting — a datetime at UTC-5 would be treated as UTC, producing the wrong timestamp
• Now uses dt.astimezone(timezone.utc) for tz-aware datetimes, preserving dt.replace() only for naive datetimes (backward compatible)
• 3 new tests covering naive, tz-aware UTC, and tz-aware non-UTC datetimes

Test plan

• Naive datetime produces same result as before (backward compat)
• Tz-aware UTC datetime produces correct timestamp
• Tz-aware non-UTC datetime (UTC-5) correctly converts
• All 51 tests pass

🤖 Generated with Claude Code

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Note

Medium Risk
Changes how timestamps are generated for tz-aware datetime values, which can alter event/customer data sent to the API for non-UTC inputs. Scope is small and covered by added unit tests, but impacts payload correctness.

Overview
Fixes datetime sanitization so timezone-aware datetime values are converted to UTC before timestamping (using astimezone(timezone.utc)) instead of silently overriding their timezone via replace().

Keeps prior behavior for naive datetimes (assumed UTC) and adds tests for naive, UTC-aware, and non-UTC-aware datetime inputs to prevent regressions.

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