fix: normalize StringToTimeLocationHookFunc error across platforms

[flohoss]

Fixes #212

What changed

wrapTimeParseLocationError now always wraps errors from time.LoadLocation with a stable message (invalid time zone format: %w), instead of only when the error text contains "unknown time zone" / starts with "time: unknown format" and leaking everything else raw.

Why

Go stdlib's loadLocation returns the first non-ENOENT error from the platform zone sources (e.g. operation not permitted / EACCES on sandboxed darwin) before falling back to the embedded time/tzdata. Only when every source is ENOENT does it emit unknown time zone <name>. Depending on the host, the hook therefore leaked raw syscall errors into decoding errors, breaking portable error handling and user-facing messages.

Behavior change detail

The wrapped error keeps its invalid time zone format: prefix, but the trailing text is now whatever time.LoadLocation returned (e.g. unknown time zone Mars/Olympus_Mons on Linux, operation not permitted on sandboxed darwin). No existing test in this repo asserted the raw syscall text; regression tests added for the raw-error path, wrapping preservation (errors.Is), and the nil passthrough.