Fix JSON Schema generation during JsonCodec derivation

[plokhotnyuk]

No description provided.

[Copilot]

Pull request overview

This PR adjusts JSON Schema generation to better reflect the actual decoding/encoding semantics of the JSON codecs (notably: tuples are fixed-length, and collections/maps treat null as empty), and improves variant/union schema output.

Changes:

• Update derived JSON Schemas for Unit, tuples, collections, and maps (e.g., maxProperties = 0, fixed-length tuples, nullable array/object unions).
• Add discriminator-related schema support for field-discriminated variants and adjust schema analysis order in JsonSchemaToReflect.
• Update test expectations to match the new generated JSON Schema shapes.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
schema/shared/src/test/scala/zio/blocks/schema/json/SchemaToJsonSchemaSpec.scala	Updates schema assertions (e.g., nullable array/object types, maxProperties = 0).
schema/shared/src/test/scala/zio/blocks/schema/json/JsonCodecToJsonSchemaSpec.scala	Mirrors updated assertions for codec-derived schemas.
schema/shared/src/test/scala/zio/blocks/schema/json/JsonTestUtils.scala	Minor local rename in test helper logic.
schema/shared/src/test/scala/zio/blocks/schema/json/JsonCodecDeriverSpec.scala	Adds safety note for string-parsed BigInt/BigDecimal decoding in tests.
schema/shared/src/main/scala/zio/blocks/schema/json/JsonSchemaToReflect.scala	Reorders schema-shape analysis to detect sequences/maps earlier.
schema/shared/src/main/scala/zio/blocks/schema/json/JsonSchema.scala	Changes withNullable handling and adds withDiscriminatorField helper.
schema/shared/src/main/scala/zio/blocks/schema/json/JsonCodecDeriver.scala	Updates JSON Schema emission for tuples/records/variants/collections/maps (titles, null unions, fixed-length tuples, discriminator injection).
schema/shared/src/main/scala/zio/blocks/schema/json/JsonCodec.scala	Adjusts Unit JSON Schema to use maxProperties = 0 and removes unused import.