fix: serialize Tool schema as parameters instead of parametersSchema

[pjperez]

Summary

The custom Serialize impl for Tool emitted the JSON key parametersSchema, but the Copilot CLI reads parameters when mapping external tool definitions. This mismatch caused every SDK-registered tool's schema to be silently dropped — the CLI fell back to an empty schema ({type: "object", properties: {}}) so the model never saw the tool's properties or required fields.

This was correct before 46f093f#diff-ed12f5ea605f23bb4d26cc65778e3daff9db3eec40e2abfc82beafca130a99a0 that introduced the change.

Root cause

In src/types.rs, the Tool serializer used:

state.serialize_field("parametersSchema", &self.parameters_schema)?;

The CLI's mapProtocolExternalTools reads n.parameters, not n.parametersSchema:

// CLI: mapProtocolExternalTools
e.map(n => ({
    name: n.name,
    description: n.description,
    parameters: n.parameters  // reads "parameters"
}))

Since n.parameters was always undefined, the CLI fell back to:

   input_schema: b.parameters ?? {type: "object", properties: {}}

Fix

One-line change — serialize the field as parameters instead of parametersSchema:

   state.serialize_field("parameters", &self.parameters_schema)?;

The Rust struct field (parameters_schema) and the builder method (.schema()) are unchanged.

Verification

• The official Python SDK also uses parameters as the serialized field name, confirming this is the expected wire format. https://github.com/github/copilot-sdk/blob/c4b3b366c4bd8dfba9ba4aa05e4019825360ad78/python/copilot/types.py#L138
• All 108 unit tests pass, plus attribution checks and doc-tests.

Impact

Every tool registered via the SDK's Tool struct had its schema silently dropped. The model only saw name and description, and had
to guess what arguments to provide. Tools with required parameters failed because the model didn't know to supply them.