Allow Client#call_tool to accept a tool name

## Motivation and Context

`Client#call_tool` previously required an `MCP::Client::Tool` object to call a tool.
This made the API harder to use compared to other MCP SDK implementations,
which accept a tool name string directly:

- Python SDK: `session.call_tool(name, arguments)` accepts a name string.
  https://github.com/modelcontextprotocol/python-sdk/blob/v1.26.0/src/mcp/client/session.py#L370
- TypeScript SDK: `client.callTool({ name, arguments })` accepts a name string.
  https://github.com/modelcontextprotocol/typescript-sdk/blob/ccb78f2/packages/client/src/client/client.ts#L834

## How Has This Been Tested?

Updated existing tests and added new tests.

## Breaking Change

No. This change adds a `name:` parameter so callers can pass a tool name string
directly without looking up a `Tool` object first. The existing `tool:` parameter
continues to work for backward compatibility.

Author: koic

lib/mcp/client.rb

@@ -92,21 +92,29 @@ def prompts
 
     # Calls a tool via the transport layer and returns the full response from the server.
     #
+    # @param name [String] The name of the tool to call.
     # @param tool [MCP::Client::Tool] The tool to be called.
     # @param arguments [Object, nil] The arguments to pass to the tool.
     # @param progress_token [String, Integer, nil] A token to request progress notifications from the server during tool execution.
     # @return [Hash] The full JSON-RPC response from the transport.
     #
-    # @example
+    # @example Call by name
+    #   response = client.call_tool(name: "my_tool", arguments: { foo: "bar" })
+    #   content = response.dig("result", "content")
+    #
+    # @example Call with a tool object
     #   tool = client.tools.first
     #   response = client.call_tool(tool: tool, arguments: { foo: "bar" })
     #   structured_content = response.dig("result", "structuredContent")
     #
     # @note
     #   The exact requirements for `arguments` are determined by the transport layer in use.
     #   Consult the documentation for your transport (e.g., MCP::Client::HTTP) for details.
-    def call_tool(tool:, arguments: nil, progress_token: nil)
-      params = { name: tool.name, arguments: arguments }
+    def call_tool(name: nil, tool: nil, arguments: nil, progress_token: nil)
+      tool_name = name || tool&.name
+      raise ArgumentError, "Either `name:` or `tool:` must be provided." unless tool_name
+
+      params = { name: tool_name, arguments: arguments }
       if progress_token
         params[:_meta] = { progressToken: progress_token }
       end

test/mcp/client_test.rb

@@ -67,6 +67,32 @@ def test_call_tool_sends_request_to_transport_and_returns_content
       assert_equal([{ type: "text", text: "Hello, world!" }], content)
     end
 
+    def test_call_tool_by_name
+      transport = mock
+      arguments = { foo: "bar" }
+      mock_response = {
+        "result" => { "content" => [{ "type": "text", "text": "Hello, world!" }] },
+      }
+
+      transport.expects(:send_request).with do |args|
+        args.dig(:request, :params, :name) == "tool1" &&
+          args.dig(:request, :params, :arguments) == arguments
+      end.returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.call_tool(name: "tool1", arguments: arguments)
+      content = result.dig("result", "content")
+
+      assert_equal([{ type: "text", text: "Hello, world!" }], content)
+    end
+
+    def test_call_tool_raises_when_no_name_or_tool
+      client = Client.new(transport: mock)
+
+      error = assert_raises(ArgumentError) { client.call_tool(arguments: { foo: "bar" }) }
+      assert_equal("Either `name:` or `tool:` must be provided.", error.message)
+    end
+
     def test_resources_sends_request_to_transport_and_returns_resources_array
       transport = mock
       mock_response = {