Raise on invalid timestamps instead of silently dropping

[hownowstephen]

Summary

• track(), track_anonymous(), and track_delivery_metric() previously silently discarded invalid timestamps (milliseconds, strings, Time objects) — now they raise ParamError with a descriptive message
• Consistent with track_push_notification_event which already raised on invalid timestamps
• Added validate_timestamp! private helper to centralize validation

Breaking change

Users relying on silent discard of invalid timestamps will now get Customerio::Client::ParamError. This is intentional — silent data loss is worse than a loud error.

Test plan

• Updated 3 tests in #track from "silently drops" to "raises ParamError"
• Updated 1 test in #track_delivery_metric from "silently drops" to "raises ParamError"
• All 130 examples pass
• RuboCop clean

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Note

Medium Risk
This changes public client behavior by turning previously ignored invalid timestamp inputs into hard errors, which is a breaking change for callers. Logic is localized to timestamp validation but affects common tracking methods.

Overview
Stops silently dropping invalid timestamp values in track/track_anonymous event creation and track_delivery_metric; invalid timestamps now raise Customerio::Client::ParamError.

Adds a shared validate_timestamp! helper and updates event creation to prefer timestamp keyword arg over attributes[:timestamp] while validating the chosen value, with specs updated to expect errors for milliseconds, non-integers, and Time objects.

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