* [Schema] Always emit {} for empty tool schema properties
Normalize empty properties arrays (including nested object schemas and
outputSchema) in Tool::__construct so tools/list never serializes
invalid JSON Schema properties: [].
Fixes#405
* [Schema] Normalize every empty sub-schema, not only properties
Walking into `items` and `additionalProperties` without normalizing an empty
sub-schema at those positions left the `[]`-instead-of-`{}` bug intact one
level down. Most notably it regressed #151 on a `Tool::fromArray()`
round-trip: `SchemaGenerator` emits `items: {}` for untyped array parameters,
which decodes to `[]` and was re-serialized as `[]` — the exact schema strict
clients reject.
Empty sub-schemas are now replaced with a `\stdClass` at every schema
position, and the keyword list is completed to cover draft-07 through
2020-12: `$defs`, `definitions`, `dependentSchemas`, `patternProperties`,
`propertyNames`, `contains`, `not`, `if`/`then`/`else`, `additionalItems`,
and `unevaluatedItems`/`unevaluatedProperties`. Keywords holding plain JSON
arrays (`allOf: []`, `enum: []`, `dependentRequired`) keep encoding as `[]`.
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Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>
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