Fix timezone-aware convertions for isotime, time, and utclong

[mbtools]

Summary

• Pass the requested timezone through unix time conversion instead of always using the default timezone.
• Rebuild ISO time strings from date/time components and preserve fractional seconds when present.
• Tighten time conversion handling for character/string inputs and separate native time handling.
• Use safer arithmetic for Unix time calculation and explicit packed-to-utclong conversion.
• Update an ABAP unit assertion to compare against an explicit xstring conversion.

Testing

• Updated and extended unit test coverage around conversion behavior.
• Not run (not requested)