Implement issue 161: Add unit tests for read_multi_modal tool

- Add test_tool_read_multi_modal.sh with tests for:
  - Successfully reading a PNG image file
  - Error handling for non-existent files
  - Reading a JPG file (different MIME type)

- Fix json_types.hpp to properly serialize std::optional and std::variant
  types by adding serialize_to_json() helper function that dispatches to
  the correct serialization method.

- Register new test in tests/CMakeLists.txt

include/json_types.hpp

@@ -244,8 +244,29 @@ struct json_to_impl<std::variant<Alternatives...>> {
         nlohmann_json_t.field = std::move(nlohmann_json_field_result.value()); \
     }
 
+// Helper to serialize a value to nlohmann::json
+// Handles std::optional, std::variant, and primitive types
+template<typename T>
+nlohmann::json serialize_to_json(const T& value) {
+    return value;  // Works for primitives via implicit conversion
+}
+
+template<typename T>
+nlohmann::json serialize_to_json(const std::optional<T>& opt) {
+    nlohmann::json j;
+    ::to_json(j, opt);
+    return j;
+}
+
+template<typename... Alternatives>
+nlohmann::json serialize_to_json(const std::variant<Alternatives...>& v) {
+    nlohmann::json j;
+    ::to_json(j, v);
+    return j;
+}
+
 // to_json using nlohmann's existing pattern
-#define DRH_JSON_TO(field) nlohmann_json_j[#field] = nlohmann_json_t.field;
+#define DRH_JSON_TO(field) nlohmann_json_j[#field] = serialize_to_json(nlohmann_json_t.field);
 
 // The main macro - generates to_json and json_to<Type>() template specialization
 #define DRH_DEFINE_TYPE_REQUIRED(Type, ...) \

tests/CMakeLists.txt

@@ -19,6 +19,7 @@ add_dr_test(test_tool_read_file)
 add_dr_test(test_tool_list_files)
 add_dr_test(test_tool_grep)
 add_dr_test(test_tool_shell)
+add_dr_test(test_tool_read_multi_modal)
 
 # Add feature validation tests
 add_dr_test(test_tool_restrictions)

tests/test_tool_read_multi_modal.sh

@@ -0,0 +1,76 @@
+#!/bin/bash
+# test_tool_read_multi_modal.sh - Tests for read_multi_modal tool
+
+# Create a minimal 1x1 PNG file for testing
+printf '\x89\x50\x4E\x47\x0D\x0A\x1A\x0A\x00\x00\x00\x0D\x49\x48\x44\x52\x00\x00\x00\x01\x00\x00\x00\x01\x08\x02\x00\x00\x00\x90\x77\x53\xDE\x00\x00\x00\x0C\x49\x44\x41\x54\x08\xD7\x63\xF8\xCF\xC0\x00\x00\x03\x01\x01\x00\x18\xDD\x8D\xB4\x00\x00\x00\x00\x49\x45\x4E\x44\xAE\x42\x60\x82' > test_image.png
+
+# Test 1: Successfully read a valid image file
+# Mock response: ask for read_multi_modal
+tool_call read_multi_modal '{"path":"test_image.png"}' >> mock_responses.txt
+be_done 'Read complete' >> mock_responses.txt
+
+OUTPUT=$("${DRHARNESS}" -F chat.json -n -m "Read image" -t read_multi_modal -d < "mock_responses.txt" 2>/dev/null)
+
+if [ "${OUTPUT}" != "Read complete" ]; then
+    echo "FAIL: read_multi_modal tool - expected 'Read complete', got '${OUTPUT}'"
+    exit 1
+fi
+
+# Verify tool result is stored as an array (Content with vector<ContentPart>)
+CONTENT_TYPE=$(dr_content_type chat.json 2)
+if [ "$CONTENT_TYPE" != "array" ]; then
+    echo "FAIL: Expected tool result content to be an array, got '${CONTENT_TYPE}'"
+    exit 1
+fi
+
+# Verify the content part type is image_url
+PART_TYPE=$(dr_content_part_type chat.json 2 0)
+if [ "$PART_TYPE" != "image_url" ]; then
+    echo "FAIL: Expected content part type to be 'image_url', got '${PART_TYPE}'"
+    exit 1
+fi
+
+# Verify the image URL is a valid PNG data URI
+IMAGE_URL=$(dr_content_part_image_url chat.json 2 0)
+if ! echo "$IMAGE_URL" | grep -q "^data:image/png;base64,"; then
+    echo "FAIL: Expected image URL to be PNG data URI, got '${IMAGE_URL}'"
+    exit 1
+fi
+
+# Test 2: Error handling for non-existent file
+rm -f chat.json
+tool_call read_multi_modal '{"path":"nonexistent_image.png"}' >> mock_responses2.txt
+be_done 'Error handled' >> mock_responses2.txt
+
+"${DRHARNESS}" -F chat.json -n -m "Read nonexistent" -t read_multi_modal -d < "mock_responses2.txt" >/dev/null 2>&1
+
+# Verify tool result is a string error message
+CONTENT_TYPE=$(dr_content_type chat.json 2)
+if [ "$CONTENT_TYPE" != "string" ]; then
+    echo "FAIL: Expected error content to be a string, got '${CONTENT_TYPE}'"
+    exit 1
+fi
+
+RESULT=$(dr_result chat.json 2)
+if ! echo "$RESULT" | grep -q "Failed to read image"; then
+    echo "FAIL: Expected error message about failed image read, got '${RESULT}'"
+    exit 1
+fi
+
+# Test 3: Read a JPG file (different MIME type)
+printf '\xFF\xD8\xFF\xE0\x00\x10JFIF' > test_image.jpg
+
+rm -f chat.json
+tool_call read_multi_modal '{"path":"test_image.jpg"}' >> mock_responses3.txt
+be_done 'JPG read complete' >> mock_responses3.txt
+
+"${DRHARNESS}" -F chat.json -n -m "Read JPG" -t read_multi_modal -d < "mock_responses3.txt" >/dev/null 2>&1
+
+# Verify the image URL is a valid JPEG data URI
+IMAGE_URL=$(dr_content_part_image_url chat.json 2 0)
+if ! echo "$IMAGE_URL" | grep -q "^data:image/jpeg;base64,"; then
+    echo "FAIL: Expected image URL to be JPEG data URI, got '${IMAGE_URL}'"
+    exit 1
+fi
+
+echo "PASS: tool_read_multi_modal"