fix(java): emit @JsonFormat for optional date/time properties

[schani]

Bug

With --datetime-provider legacy, the Java (Jackson) renderer attaches
@JsonFormat(pattern = ...) to date/date-time/time properties that
are required, but silently omits it for the same kind of properties
when they are optional — producing inconsistent serialization/parsing
behavior between the two, even though both hold the same value type.

Repro:

node dist/index.js --lang java --src-lang schema --lombok --datetime-provider legacy demo.schema

with a schema containing both a required and an optional date-time
property. Before the fix, only the required property got @JsonFormat;
after the fix, both do.

Root cause

JavaJacksonRenderer.annotationsForAccessor switched directly on
p.type.kind to decide which @JsonFormat annotation to emit. A
required date/date-time/time property has that kind directly on its
type, so the switch matched. An optional property is represented
internally as a nullable union (date-time | null), so p.type.kind is
"union" and none of the cases matched, silently skipping the
annotation. This was invisible with the default java8 datetime
provider because that provider's annotation lists are empty regardless.

Fix

Unwrap a nullable union to its non-null member before switching on the
kind, using the same nullableFromUnion pattern already used elsewhere
in the Java renderer (e.g. JavaRenderer.javaType). Minimal, four-line
change in packages/quicktype-core/src/language/Java/JavaJacksonRenderer.ts.

Test coverage

Added test/inputs/schema/optional-date-time.schema (+
.1.json sample) with matching required and optional date, time, and
date-time properties. This schema is picked up automatically by the
existing schema-java-datetime-legacy fixture (JavaLanguageWithLegacyDateTime
in test/languages.ts, registered in test/fixtures.ts), and is not
excluded by that language's skipSchema list.

Verification done locally

• npm run build passes.
• Re-ran the original repro: optionalDate/requiredDate now get
  identical @JsonFormat annotations under --datetime-provider legacy.
• Confirmed the default java8 provider still emits zero @JsonFormat
  annotations (unaffected/expected behavior).
• Spot-checked code generation for the new schema file across several
  other languages (C#, Python, Go, Rust, Swift, Kotlin, TypeScript) to
  confirm the new fixture input doesn't break unrelated languages.
• Maven is not available in this sandbox, so the full compile-and-run
  fixture (FIXTURE=schema-java-datetime-legacy script/test) could not
  be executed end-to-end locally; CI will run it.

Fixes #1593

🤖 Generated with Claude Code