fix: updating schema to match the json schema protocol

[pauldotknopf]

https://json-schema.org/draft-07/schema

"$defs" should be "definitions"

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[darkhaniop]

The tool used to generate a2a.json probably used the latest version draft 2020-12. The key definitions was replaced with $defs in 2019-09 (predecessor of 2020-12).

From https://json-schema.org/draft/2019-09/draft-handrews-json-schema-validation-02#rfc.appendix.A

"definitions"
Renamed to "$defs" to match "$ref" and be shorter to type. Schema vocabulary authors SHOULD NOT define a "definitions" keyword with different behavior in order to avoid invalidating schemas that still use the older name. While "definitions" is absent in the single-vocabulary meta-schemas referenced by this document, it remains present in the default meta-schema, and implementations SHOULD assume that "$defs" and "definitions" have the same behavior when that meta-schema is used.

If that's the case, the only line that needs to be corrected is the $schema specifier from

A2A/specification/json/a2a.json

Line 2 in 175cecd

"$schema": "http://json-schema.org/draft-07/schema#",

to:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  ...
}

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Edit: add a quote from the referenced document