Function parseTimeInTimezone

Interpret a naive (offset-less) date/time string as local-to-the-given-timezone and return an ISO 8601 string with the correct offset.

If the input already includes an explicit offset or Z, it is returned as-is.

Supports the following input formats for naive strings:

• "YYYY-MM-DDTHH:mm[:ss]" (ISO bare)
• "YYYY-MM-DD HH:mm[:ss]" (ISO bare with space)
• "MM/DD/YYYY HH:mm[:ss]" (US format, used by Cedar Fair)