Use the parsed offset in ZonedDateTime::parse() when a region is present

[gnutix]

Item 1 of #137, with the stricter behaviour suggested there.

Problem

ZonedDateTime::from() parsed the offset, then discarded it and resolved the local date-time against the region again:

$z = ZonedDateTime::ofInstant(Instant::of(1729989000), TimeZone::parse('Europe/Zurich'));
(string) $z;                                      // 2024-10-27T02:30:00+02:00[Europe/Zurich]
ZonedDateTime::parse((string) $z)->isEqualTo($z); // false: +01:00, one hour later

ZonedDateTime::parse('2024-06-01T12:00Z[Europe/Zurich]'); // 12:00+02:00, a different instant

Change

• When a region is present, the instant is computed from the local date-time and the parsed offset, then converted to the region.
• If the resulting offset differs from the parsed one, a DateTimeException is thrown:
  • The offset Z is not valid for 2024-06-01T12:00 in Europe/Zurich. when the region doesn't use that offset at that time;
  • The local date-time 2024-03-31T02:30 does not exist in Europe/Zurich. when the local date-time falls in a DST gap.
• Both occurrences of an overlap now parse to the right instant, so toString() → parse() round-trips.

This is a behaviour change: input with an offset that doesn't match the region used to be accepted silently, and now throws.

One existing fixture (ZonedDateTimeTest, testPlusPeriod/testMinusPeriod data set #11) used -07:00 for America/Los_Angeles in February. It only passed because the offset was ignored, and now uses -08:00; the expected result is unchanged.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Fm9bvgKFasEGzpK48bQrxA

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.24%. Comparing base (44f8c01) to head (84e568d).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #140   +/-   ##
=========================================
  Coverage     99.24%   99.24%           
- Complexity     1141     1143    +2     
=========================================
  Files            48       48           
  Lines          2501     2517   +16     
=========================================
+ Hits           2482     2498   +16     
  Misses           19       19           

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[BenMorel]

Thank you, @gnutix!