Fix UTC guard in from_timestamp to skip redundant timezone conversion

[AhmedAli58]

What

Fix from_timestamp() UTC guard condition so UTC inputs do not perform an unnecessary in_timezone() call.\n\n## Why\nThe previous condition used or, which is always true for this check and forced conversion even when tz was already UTC.\n\n## How\n- Change if tz != utc or tz != "UTC" to if tz != utc and tz != "UTC"\n- Add regression tests to ensure UTC object and "UTC" string skip timezone conversion\n\n## Verification\nPYTHONPATH=shared/timezones/src python3 -m pytest -q shared/timezones/tests\n- 32 passed

[boring-cyborg]

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