feat(kotlin): Support for date and datetime from JSON schema

[nikhilunni]

Description

Add support for JSON Schema format: date-time, format: date, and format: time in the Kotlin code generator. These formats now generate proper java.time.* types instead of String.

This implementation follows the same pattern as Java's date-time support, using:

• OffsetDateTime for format: "date-time"
• LocalDate for format: "date"
• OffsetTime for format: "time"

The feature works across all Kotlin frameworks: just-types, jackson, klaxon, and kotlinx.

Related Issue

Fixes #2460

Motivation and Context

We prefer Kotlin generation to Java generation for nullability propagation, but did not support datetime formatting, as Java does. Plus, saw there was a longstanding issue for this.

Previous Behaviour / Output

For the given JSON schema:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "eventDate": {
      "type": "string",
      "format": "date"
    },
    "eventTime": {
      "type": "string",
      "format": "time"
    },
    "plannedDateTime": {
      "type": "string",
      "format": "date-time"
    }
  }
}

It would output:

package quicktype

data class MySchema (
    val eventDate: String,
    val eventTime: String,
    val plannedDateTime: String
)

New Behaviour / Output

package quicktype

import java.time.OffsetDateTime
import java.time.LocalDate
import java.time.OffsetTime

data class MySchema (
    val eventDate: LocalDate,
    val eventTime: OffsetTime,
    val plannedDateTime: OffsetDateTime
)

How Has This Been Tested?

Manual Testing

• Tested with the existing test/inputs/schema/date-time.schema file
• Verified output for all Kotlin frameworks:
  • --framework just-types
  • --framework jackson
  • --framework klaxon
  • --framework kotlinx
• Tested with the exact schema from issue Kotlin date and datetime types are generated as strings from json schema #2460
• Verified proper handling of:
  • Simple date-time properties
  • Date-time types in unions
  • Date-time types in arrays
  • Complex nested structures with date-time types

Automated Testing

• Added "date-time" feature to KotlinLanguage test configuration
• Added "date-time" feature to KotlinJacksonLanguage test
  configuration
• Follows the same pattern as the Golang date-time feature
  implementation (commit 1f89a97)