Fix client methods silently swallowing JSON-RPC error responses

## Motivation and Context

Client methods (`tools`, `resources`, `resource_templates`, `prompts`, `read_resource`, `get_prompt`, `call_tool`)
silently swallowed JSON-RPC error responses by falling back to empty defaults. This did not conform to JSON-RPC 2.0,
which clearly distinguishes error responses from success responses.
Both the Python SDK (`MCPError`) and TypeScript SDK (`ProtocolError`) raise exceptions on error responses.
The Ruby SDK was the only one silently discarding them.

Introduces a common `request` method that handles JSON-RPC request construction and error checking in one place,
replacing the duplicated `transport.send_request` calls across all client methods.

## How Has This Been Tested?

Added tests for each affected method verifying that `ServerError` is raised with the correct `code`, `message`,
and `data` attributes.

## Breaking Changes

The previous behavior of silently returning empty defaults on server errors did not conform to JSON-RPC 2.0.
Error responses should be surfaced to callers, not swallowed.
Callers that relied on the silent fallback behavior will need to rescue `MCP::Client::ServerError`.

test/mcp/client_test.rb

@@ -369,5 +369,92 @@ module MCP
       client = Client.new(transport: transport)
       client.call_tool(tool: tool, arguments: arguments)
     end
+
+    def test_tools_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_601, "message" => "Method not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.tools }
+      assert_equal(-32_601, error.code)
+      assert_equal("Method not found", error.message)
+    end
+
+    def test_resources_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_602, "message" => "Invalid params" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.resources }
+      assert_equal(-32_602, error.code)
+    end
+
+    def test_read_resource_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_602, "message" => "Resource not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      assert_raises(Client::ServerError) { client.read_resource(uri: "file:///missing") }
+    end
+
+    def test_get_prompt_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_602, "message" => "Prompt not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      assert_raises(Client::ServerError) { client.get_prompt(name: "missing") }
+    end
+
+    def test_prompts_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_601, "message" => "Method not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      assert_raises(Client::ServerError) { client.prompts }
+    end
+
+    def test_resource_templates_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_601, "message" => "Method not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      assert_raises(Client::ServerError) { client.resource_templates }
+    end
+
+    def test_call_tool_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_602, "message" => "Tool not found" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.call_tool(name: "missing") }
+      assert_equal(-32_602, error.code)
+    end
+
+    def test_server_error_includes_data_field
+      transport = mock
+      mock_response = {
+        "error" => { "code" => -32_603, "message" => "Internal error", "data" => "extra details" },
+      }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.tools }
+      assert_equal("extra details", error.data)
+    end
   end
 end