Summary

Fix tool not found errors to return JSON-RPC Error instead of JSON-RPC Result with isError: true, per MCP specification.

Problem

When a client calls a nonexistent tool, the current implementation returns:

{
  "jsonrpc": "2.0",
  "id": "3",
  "result": {
    "content": [
      {
        "type": "text",
        "text": "MCP error -32602: Tool nonexistent_tool not found"
      }
    ],
    "isError": true
  }
}

But the MCP specification requires:

{
  "jsonrpc": "2.0",
  "id": 3,
  "error": {
    "code": -32602,
    "message": "Unknown tool: nonexistent_tool"
  }
}

Root Cause

The tools/call handler's catch block only re-threw UrlElicitationRequired errors. All other ProtocolError instances (including tool not found, tool disabled, invalid params, etc.) were caught and wrapped in a CallToolResult with isError: true.

Fix

Changed the catch block to re-throw all ProtocolError instances, not just UrlElicitationRequired. This ensures protocol-level errors are returned as JSON-RPC errors per the spec.

Tests

Updated the existing test for nonexistent tools to expect a thrown error instead of a CallToolResult with isError: true.

Fixes #1510