GH-54534: [C++] Fix truncation of sub-minute UTC offsets when formatting and parsing zoned timestamps

[ShivanshhhG]

Rationale for this change

When formatting zoned timestamps whose UTC offset includes sub-minute seconds (e.g. historical IANA zones such as Africa/Monrovia prior to 1972 at UTC-00:44:30 or Europe/Paris prior to 1911 at UTC+00:09:21), the vendored date.h library truncated the offset to whole minutes via duration_cast<minutes>, dropping the remaining seconds and emitting an inaccurate offset (e.g. -0044 instead of -004430). When parsed back (via Cast or CSV reader), the timestamp instant was shifted by up to 59 seconds.

What changes are included in this PR?

1. cpp/src/arrow/vendored/datetime/date.h: In to_stream for %z and %Ez, check whether *offset_sec has non-zero seconds (s != 0). If present, format the seconds component (±hhmmss for %z, ±hh:mm:ss for %Ez). If whole minutes, preserve the standard ±hhmm / ±hh:mm representation.
2. cpp/src/arrow/util/value_parsing.h: Add detail::ParseHHMMSS to parse 6-digit offsets and update ParseTimestampISO8601 to recognize and parse [+-]HHMMSS and [+-]HH:MM:SS zone offsets.
3. cpp/src/arrow/util/value_parsing.cc & cpp/src/arrow/compute/kernels/scalar_temporal_unary.cc: Recognize %Ez as providing a zone offset in StrptimeTimestampParser and GetZone.
4. cpp/src/arrow/util/value_parsing_test.cc: Add unit tests for sub-minute offsets (+004430, +00:44:30, -004430, -00:44:30) and invalid offset validations (+00:00:60, +000060, +240000, +24:00:00).
5. cpp/src/arrow/compute/kernels/scalar_cast_test.cc: Add regression tests casting zoned timestamps in Africa/Monrovia (-00:44:30) and Europe/Paris (+00:09:21) to string arrays.

Are these changes tested?

Yes, verified with standalone compilation and unit tests covering parsing, formatting, validation bounds, and cast kernels.

Are there any user-facing changes?

Formatting zoned timestamps with historical sub-minute UTC offsets now accurately emits the seconds component (±HH:MM:SS or ±HHMMSS) rather than truncating them to minutes, preserving instant fidelity across round-trips.

• GitHub Issue: [C++] Casting a zoned timestamp to string (and the CSV writer) truncates UTC offsets that have seconds, so the written instant is wrong #54534

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