fix(codecompanion): add additionalProperties to tool_input of use_mcp

[misumisumi]

Description

Following error cause using GPT-5.x of github copilot on codecompanion.

[ERROR] 2026-01-15 19:50:48
Error: {"error":{"message":"Invalid schema for function 'use_mcp_tool': In context=('properties', 'tool_input', 'type', '0'), 'additionalProperties' is required to be supplied and to be false.","code":"invalid_request_body"}}
[ERROR] 2026-01-15 19:50:48
[chat::_submit_http] Error: {
  body = "{\"error\":{\"message\":\"Invalid schema for function 'use_mcp_tool': In context=('properties', 'tool_input', 'type', '0'), 'additionalProperties' is required to be supplied and to be false.\",\"code\":\"invalid_request_body\"}}",
  exit = 0,
  headers = { "content-security-policy: default-src 'none'; sandbox", "content-type: text/plain; charset=utf-8", "strict-transport-security: max-age=31536000", "x-content-type-options: nosniff", "x-request-id: 00000-e149250b-593b-4265-a5d7-c0adc069495a", "date: Thu, 15 Jan 2026 10:50:48 GMT", "content-length: 219", "x-github-backend: Kubernetes", "x-github-request-id: F3C9:2D4625:2114F0:256D7F:6968C687" },
  status = 400
}

The error occurs because tool_input is an object. olimorris/codecompanion.nvim#2334 supports strict mode for nested objects, and it works correctly by simply adding additionalProperties.

Related Issue(s)

• Fix [Bug]: Copilot w/ GPT-5 series models fails with "Invalid schema" for several tools olimorris/codecompanion.nvim#2314 (comment)

Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I've updated the README and/or relevant docs pages
• I've run make test to ensure all tests pass
• I've run make format to format the code
• I've run make docs to update the vimdoc pages

[ravitemer]

Thanks for fixing this!