[jd-lara]

Summary

The OpenAPI Format Registry
defines date-time-local as an RFC 3339 date-time without a time zone. OpenAPI.jl did not
map it, so the field became a String. Under datetime = :zoned, a date-time with no
offset decoded as UTC, which RFC 3339 does not allow.

This PR replaces #134 (closed), which proposed a custom datetime = :as_written mode.
It uses the registry format instead, so the schema states the form of each timestamp.

Behavior

Format	datetime = :utc (default)	datetime = :zoned
date-time	Dates.DateTime, offset normalized to UTC (unchanged)	ZonedDateTime; Z or an offset is now required
date-time-local	String (unchanged)	Dates.DateTime, a wall clock with no offset
anyOf of both	wrapper struct, as before	wrapper whose value is either type; the branch is chosen by schema

Under :utc, Dates.DateTime already means a UTC instant, so date-time-local stays a
String there. Under :zoned, date-time is a ZonedDateTime, so a Dates.DateTime
can only mean the wall clock.

Changes

• src/planning.jl: format: date-time-local maps through _local_datetime_type!,
  dispatched on Val(datetime).
• src/client.jl, src/servergen.jl: _emit_model takes an encode keyword. A :zoned
  module encodes model fields, aliases, union values and additional properties through
  _encode_zoned. :utc output does not change.
• src/runtime.jl: _encode_zoned(::Dates.DateTime) writes yyyy-mm-ddTHH:MM:SS.sss
  with no offset, and recurses into vectors, tuples, dicts and named tuples. Any other
  value goes to _encode.
• ext/OpenAPITimeZonesExt.jl: the ZonedDateTime decoder requires Z or an offset.
  Its regex ends with \z, so a trailing newline is rejected. PCRE $ matches before
  one.
• CONTRACT_VERSION is 5, because generated code imports _encode_zoned. Contract 3 and
  4 modules still load.
• Docs: clients.md, models.md, artifacts.md, servers.md, and the client
  docstring.

Breaking change for :zoned users

A :zoned module rejects a date-time value with no offset. Before, it read the value as
UTC. The existing test that pinned that behavior now asserts a DecodeError. A producer
that writes zoneless values should declare them date-time-local.

Limits

A date-time-local value sent directly as a query, header or cookie parameter, or as a
top-level request body, still goes through _encode / _scalar, which add Z. Only
model values use _encode_zoned.

The release that carries contract 5 needs a version bump. I left that to you.

Tests

test/runtime.jl has a new test set, "date-time-local is a wall clock under datetime =
:zoned". It covers:

• :utc keeps String;
• :zoned field types for a required, an array and an anyOf field;
• decode and encode of each form;
• anyOf branch selection by schema pattern;
• rejection of an offset on a wall-clock field.

The :zoned test set now asserts that a zoneless value and a trailing newline are
rejected. test/generated_contract.jl moves its tripwire to contract 5 and checks that
contracts 3, 4 and 5 load.

julia --project=. -e 'using Pkg; Pkg.test()': "OpenAPI tests passed", 1442 passes, 0
failures, 0 errors. generated runtime units: 392 of 392.