fix: emit full seconds in Instant and ZonedDateTime toISOString (RFC 3339)

[dualfroz]

Problem

Fixes #110

Problem

Instant::toISOString() (and therefore Instant::jsonSerialize() / (string)) and
ZonedDateTime::toISOString() emit a compact time format that omits the seconds when they
are zero, e.g.:

2023-11-14T22:14Z            // whole minute
1970-01-01T00:00Z            // epoch
2023-11-14T23:00Z            // whole hour

This is a valid ISO 8601 time, but it is not a valid RFC 3339 date-time. RFC 3339
section 5.6 defines full-time as partial-time time-offset where
partial-time = time-hour ":" time-minute ":" time-second [time-secfrac] — the seconds
component is mandatory, even when it is 00. Instant and ZonedDateTime are the types
meant for interoperability (JSON serialization, wire formats), so they should always emit a
value other systems can parse as RFC 3339. Producing 2023-11-14T22:14Z breaks strict
RFC 3339 parsers.

Root cause

The compact behavior is intentional and documented in
src/LocalTime.php:639 (toISOString() — "the shortest that outputs the full value of the
time"). That is correct for LocalTime and is left untouched.

The bug is that the interop types inherit that compact form:

• src/ZonedDateTime.php:740 toISOString() built the string from
  $this->localDateTime, whose __toString() delegates to LocalTime::toISOString()
  (the compact formatter).
• src/Instant.php:362 toISOString() returns (string) ZonedDateTime::ofInstant(...),
  so it inherited the same compact time. jsonSerialize() and __toString() both route
  through toISOString().

The fix

ZonedDateTime::toISOString() now builds the time portion explicitly with a mandatory
HH:MM:SS component (keeping the existing optional fractional-seconds handling), instead of
delegating to LocalTime's compact formatter. Instant::toISOString() is fixed
transitively because it delegates to ZonedDateTime; its PHPDoc now documents the RFC 3339
guarantee. LocalTime is deliberately not changed.

Output after the fix:

2023-11-14T22:14:00Z
1970-01-01T00:00:00Z
2023-11-14T23:00:00Z
2023-11-14T23:14:00+01:00[Europe/Paris]

Fractional seconds and non-zero seconds are unchanged (e.g.
1970-01-01T00:00:00.000123456Z, 1969-12-31T23:59:59Z).

Why the test fixture changes are justified (BC break)

Some existing fixtures pinned the buggy compact output and had to be updated to the
RFC-3339-correct strings — this is the legitimate case of changing a maintainer's tests
because the old expected values encode the bug being fixed:

• tests/InstantTest.php providerToString: [0, 0, '1970-01-01T00:00Z'] ->
  '1970-01-01T00:00:00Z'.
• tests/ZonedDateTimeTest.php: the DST plus/minus and interval result strings that landed
  on whole minutes/hours (e.g. 2025-03-30T01:00+01:00[Europe/Prague] ->
  2025-03-30T01:00:00+01:00[Europe/Prague]).
• tests/IntervalTest.php: one exception-message fixture (1970-01-04T11:20Z ->
  1970-01-04T11:20:00Z).

New assertions were added for whole-minute and whole-hour values to lock in the :00:

• tests/InstantTest.php: [1700000040, 0, '2023-11-14T22:14:00Z'] and
  [1700002800, 0, '2023-11-14T23:00:00Z'].
• tests/ZonedDateTimeTest.php: ['2000-01-20T12:34', '-07:00', '2000-01-20T12:34:00-07:00']
  and ['2000-01-20T12:00', 'America/Los_Angeles', '2000-01-20T12:00:00-08:00[America/Los_Angeles]'].

Because the serialized string representation changes, this is technically a backward
compatibility break, matching the maintainer's note on issue #110: "this will target
version 0.8.0 as this is technically a BC break." Parsing is unaffected — the parser still
accepts the compact input form, so round-tripping and existing stored values keep working.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.30%. Comparing base (66dab53) to head (9dd1445).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #131   +/-   ##
=========================================
  Coverage     99.30%   99.30%           
- Complexity     1109     1113    +4     
=========================================
  Files            48       48           
  Lines          2445     2458   +13     
=========================================
+ Hits           2428     2441   +13     
  Misses           17       17           

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[BenMorel]

Input does not need to be modified:

Suggested change

	['2025-03-30T01:30:00+01:00[Europe/Prague]', 3000, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01:00[Europe/Prague]', 6000, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
	['2025-03-30T03:20:00+02:00[Europe/Prague]', -3000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T04:10:00+02:00[Europe/Prague]', -6000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01[Europe/Prague]', 3000, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01[Europe/Prague]', 6000, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
	['2025-03-30T03:20:00+02[Europe/Prague]', -3000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T04:10:00+02[Europe/Prague]', -6000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],

[BenMorel]

Input does not need to be modified:

Suggested change

	['2025-03-30T01:30:00+01:00[Europe/Prague]', 50, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01:00[Europe/Prague]', 100, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
	['2025-03-30T03:20:00+02:00[Europe/Prague]', -50, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T04:10:00+02:00[Europe/Prague]', -100, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01[Europe/Prague]', 50, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
	['2025-03-30T01:30:00+01[Europe/Prague]', 100, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
	['2025-03-30T03:20:00+02[Europe/Prague]', -50, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
	['2025-03-30T04:10:00+02[Europe/Prague]', -100, '2025-03-30T01:30:00+01:00[Europe/Prague]'],

[BenMorel]

2 nitpicks, otherwise LGTM @dualfroz!