feat(schema): add support for JSON Schema $defs and definitions (#146)

Added support for $defs and definitions properties in JsonSchema record to handle JSON Schema references properly. Added tests to verify both formats work correctly.

The JsonSchema test approach uses serialization/deserialization round-trip validation instead of property-by-property assertions. This makes tests more maintainable and less likely to break when new properties are added.

🤖 Generated with [Claude Code](https://claude.ai/code)

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Co-authored-by: Claude <noreply@anthropic.com>

mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java

@@ -703,7 +703,9 @@ public final class McpSchema {
 		@JsonProperty("type") String type,
 		@JsonProperty("properties") Map<String, Object> properties,
 		@JsonProperty("required") List<String> required,
-		@JsonProperty("additionalProperties") Boolean additionalProperties) {
+		@JsonProperty("additionalProperties") Boolean additionalProperties,
+		@JsonProperty("$defs") Map<String, Object> defs,
+		@JsonProperty("definitions") Map<String, Object> definitions) {
 	} // @formatter:on
 
 	/**