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.
  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.

test/mcp/client_test.rb

@@ -67,6 +67,32 @@ module MCP
       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 = {