Protocol errors still returned as tool execution errors (invalid arguments, server errors)

[sqrlington]

Describe the bug
Two of the three protocol error cases defined by the spec are still returned as tool execution errors (isError: true results) instead of JSON-RPC error responses:

1. Invalid arguments — missing required arguments and schema validation failures both return isError: true
2. Server errors — internal exceptions during tool execution return isError: true

This is the same class of bug as #228, which was fixed for unknown tool calls in PR #231. The other two cases were not addressed.

To Reproduce

1. Define a tool with a required argument
2. Call it with the argument missing

server = MCP::Server.new(tools: [MyTool])
response = server.handle({
  jsonrpc: "2.0",
  id: 1,
  method: "tools/call",
  params: { name: "my_tool", arguments: {} }
})

3. Response is a successful JSON-RPC result with isError: true:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "content": [{"type": "text", "text": "Missing required arguments: required_argument"}],
    "isError": true
  }
}

Same behavior for invalid schema validation and internal server errors.

Expected behavior

A JSON-RPC protocol error, consistent with how unknown tool calls are handled:

{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32602,
    "message": "Invalid params",
    "data": "Missing required arguments: required_argument"
  }
}

Additional context

The MCP spec draws a clear line between protocol errors and tool execution errors:

Protocol Errors: Standard JSON-RPC errors for issues like:

• Unknown tools
• Invalid arguments
• Server errors

Tool Execution Errors: Reported in tool results with isError: true:

• API failures
• Invalid input data
• Business logic errors

A note on server errors: #159 raised a valid concern that tool execution errors like ActiveRecord::RecordNotFound shouldn't be protocol errors. But the fix for that is for the tool to catch those exceptions and return Tool::Response.new(content, error: true) — that's the tool reporting a known failure. If an exception bubbles up uncaught, the server can't distinguish a business error from an actual crash, so treating it as a server error (protocol error) is the correct default.

The trail:

1. PR Fix tool error handling to follow MCP spec #165 moved all three protocol error cases from RequestHandlerError (JSON-RPC errors) to error_tool_response (isError: true)
2. Wrong kinds of MCP errors returned in some cases #228 flagged this as a spec violation
3. PR Return JSON-RPC protocol errors for unknown tool calls #231 fixed unknown tools only — invalid arguments and server errors still return isError: true

The remaining two cases in Server#call_tool need to raise RequestHandlerError instead of returning error_tool_response, matching the pattern PR #231 restored for unknown tools.