Fix: Support additionalProperties as both boolean and schema object

[benkaiser]

Problem

JsonObject.fromJson() hard-casts additionalProperties as bool?:

additionalProperties: json['additionalProperties'] as bool?,

In https://json-schema.org/understanding-json-schema/reference/object#additionalproperties, additionalProperties can be either a boolean (true/false) or a schema object constraining the shape of extra properties. When an MCP server returns a tool schema where additionalProperties is an object (e.g., {}), the parser throws:

type '_Map<String, dynamic>' is not a subtype of type 'bool?' in type cast

This crashes tools/list parsing entirely, preventing the client from connecting to the server.

Reproduction

Any MCP server whose tools use Zod's z.record() will trigger this. For example, the
https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/debug-server from the official ext-apps repo defines:

z.object({
  config: z.record(z.string(), z.unknown()),
  // ...
})

Which produces the JSON Schema:

{
  "type": "object",
  "properties": {
    "config": {
      "type": "object",
      "additionalProperties": {}
    }
  }
}

The additionalProperties: {} is a valid schema object (accepts any value), but mcp_dart crashes trying to cast it as
bool?.

Fix

• Changed JsonObject.additionalProperties from bool? to Object?, which can hold either a bool or a JsonSchema
• fromJson() now checks the runtime type: bool values are stored directly, Map values are parsed as JsonSchema.fromJson()
• toJson() serializes JsonSchema values back via .toJson(), booleans are written directly
• JsonSchemaValidator now validates extra properties against the schema when additionalProperties is a JsonSchema
  (previously it had a TODO comment acknowledging this was missing)

Notes

This PR was AI-assisted (Claude). I did review the code myself and also validated it worked end-to-end on the stated debug server from the official examples.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.49%. Comparing base (8a4bfb3) to head (8882640).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
+ Coverage   78.45%   78.49%   +0.04%     
==========================================
  Files          40       40              
  Lines        5371     5381      +10     
==========================================
+ Hits         4214     4224      +10     
  Misses       1157     1157              

Flag	Coverage Δ
unittests	78.49% <100.00%> (+0.04%)	⬆️

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[leehack]

Thanks for the contribution @benkaiser