feat: include YearMonth in SimpleTypeModule.forPrimitiveAndAdditionalTypes()

[SAY-5]

Resolves #562.

SimpleTypeModule.forPrimitiveAndAdditionalTypes() mapped LocalDate, LocalDateTime, ZonedDateTime, OffsetDateTime, Instant, LocalTime, OffsetTime, Duration, Period, ZoneId, etc. but not java.time.YearMonth. Jackson's JavaTimeModule serializes YearMonth as a string (e.g. "2025-10"), so the generated schema reported object instead of string.

YearMonth is added alongside ZoneId via withStringType, since RFC3339 does not define a year-month standard format. Parameterized test entry added to SchemaGeneratorSimpleTypesTest.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[CarstenWickner]

Thank you for the contribution @SAY-5 😃