Add JsonNullable support to JSON schema generator

[jewoodev]

Summary

Adds JsonNullable<T> support to JsonSchemaGenerator, so generated JSON Schemas can model values that may be omitted separately from values explicitly set to null — for both tool inputs and typed schemas.

• jackson-databind-nullable is an <optional> dependency; JsonNullableSchemaModule registers only when JsonNullable is on the classpath.
• A single CustomDefinitionProvider unwraps JsonNullable<T> at any position (tool parameters and record/POJO fields) — generating T's schema and adding "null" to the allowed types.
• Unannotated JsonNullable<T> is treated as not required by default — at both the tool parameter and record/POJO field level — while explicit @ToolParam, @JsonProperty, or @Schema settings still take precedence.

The required default intentionally differs from the generator's PROPERTY_REQUIRED_BY_DEFAULT = true for the unannotated case only — JsonNullable exists precisely so a caller can omit a value, so not-required is the natural fit. Happy to switch to an explicit opt-out if you'd rather keep required-by-default for this wrapper too. The checkstyle suppression is scoped to JsonNullableSchemaModule because the bannedNullabilityImports rule false-positives on the JsonNullable class name.

Testing

Added JUnit coverage in JsonSchemaGeneratorTests for scalar/list/object JsonNullable<T> at top-level parameters and record fields, plus explicit required-policy overrides. ./mvnw package passes locally.

Closes #6046