[jd-lara]

Summary

This PR adds a third value for the datetime option, :as_written. With :utc and
:zoned, a date-time with no offset decodes as UTC. Some producers write a wall
clock with no offset, and that value names no instant. Reading it as UTC gives a
wrong instant without an error.

With datetime = :as_written:

Input	Decoded value	Encoded output
2024-01-01T00:00:00	Dates.DateTime	2024-01-01T00:00:00.000
2024-01-01T00:00:00Z	ZonedDateTime, UTC	2024-01-01T00:00:00.000+00:00
2024-01-01T02:00:00+02:00	ZonedDateTime, FixedTimeZone +02:00	2024-01-01T02:00:00.000+02:00

The field type is Union{Dates.DateTime,TimeZones.ZonedDateTime}. The generated
module has using TimeZones, as with :zoned.

Changes

• src/planning.jl: the option check accepts :as_written. The field type comes
  from methods on Val(datetime).
• src/runtime.jl: _decode_union sends a DateTime/ZonedDateTime union to
  _decode_as_written, after the nothing check. Trial decoding would convert an
  offset to UTC. _encode_as_written(::Dates.DateTime) writes no offset, and recurses
  into vectors and dicts.
• ext/OpenAPITimeZonesExt.jl: _decode_as_written uses the same regex, millisecond
  truncation and DecodeError messages as the other decoders. It dispatches on the
  Z and offset captures. _encode_as_written(::ZonedDateTime) uses _encode.
• src/client.jl, src/servergen.jl: _emit_model takes an encode keyword. Under
  :as_written, model fields, aliases, union values and additional properties encode
  through _encode_as_written. _encode(::Dates.DateTime) does not change, so :utc
  output still has Z.
• docs/src/clients.md, docs/src/models.md: the new mode.

Limits

A date-time sent directly as a query, header or cookie parameter, or as a top-level
request body, still goes through _encode. Only model values use
_encode_as_written.

Tests

The new test set in test/runtime.jl covers:

• decode of a zoneless value, Z, +02:00, -07:00, fractional seconds, and invalid
  text;
• encode and round trip of each form;
• a generated model with required, optional, nullable and array date-time fields;
• that :utc and :zoned output does not change.

julia --project=. -e 'using Pkg; Pkg.test()': all test sets pass (1467 passes, 0
failures, 0 errors). generated runtime units: 418 of 418.

[jd-lara]

Closed. A new PR will add support for the registry format date-time-local instead of a custom datetime = :as_written mode.