[leoafarias]

Why

Stacked on #216 (leoafarias/json-schema-import-builtin-formats).

_JsonSchemaCompiler in 1.7.x already supports Draft-07 definitions alongside Draft 2020-12 $defs (and schema.toJsonSchema() emits Draft-07 shapes), but _JsonSchemaCompiler._index rejected documents that explicitly declared the Draft-07 $schema URI (http(s)://json-schema.org/draft-07/schema(#)) with unsupported_dialect. In addition, listKeywords in _JsonSchemaCompiler still contained an unused 'prefixItems' entry even though prefixItems validation is deferred to 2.0 (#171).

Scope

• Accepts http(s)://json-schema.org/draft-07/schema(#) alongside https://json-schema.org/draft/2020-12/schema(#) in _JsonSchemaCompiler.supportedSchemaDialects so Draft-07 schemas using Ack's supported subset import cleanly without stripping $schema.
• Removes the dead 'prefixItems' entry from _JsonSchemaCompiler.listKeywords so prefixItems fails directly with unsupported_keyword without being indexed as a child schema list.
• Updates docs/guides/json-schema-integration.mdx and packages/ack/CHANGELOG.md (## Unreleased).

Verification

• fvm dart analyze in packages/ack reports no issues.
• fvm dart test test/json_schema_import_test.dart test/json_schema_import_api_test.dart test/json_schema_import_conformance_test.dart passes.