feat(api): add file listing endpoint

- Add GET /api endpoint to list all markdown files
- Filter to only include .md files
- Return JSON response with files array
- Add comprehensive tests for file listing functionality

backend/tests/file_listing_test.go

@@ -0,0 +1,141 @@
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/evanreichard/markdown-editor/internal/api"
+	"github.com/evanreichard/markdown-editor/internal/logger"
+	"github.com/gorilla/mux"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFileListing(t *testing.T) {
+	dataDir, err := setupTestDir()
+	if err != nil {
+		t.Fatalf("Failed to create test dir: %v", err)
+	}
+	defer cleanupTestDir(dataDir)
+
+	// Create some test files
+	testFiles := []string{
+		"file1.md",
+		"file2.md",
+		"file3.md",
+	}
+	
+	for _, filename := range testFiles {
+		filepath := filepath.Join(dataDir, filename)
+		content := "# " + filename + "\n\nContent of " + filename
+		if err := os.WriteFile(filepath, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to create test file %s: %v", filename, err)
+		}
+	}
+
+	log := logger.NewLogger()
+	handler := api.NewAPIHandler(dataDir, log)
+	router := mux.NewRouter()
+	router.Handle("/api", handler).Methods("GET")
+	router.Handle("/api/{filename:.+\\.md}", handler)
+
+	// Test GET /api (list files)
+	req := httptest.NewRequest("GET", "/api", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+	assert.Equal(t, http.StatusOK, w.Code)
+	
+	var response struct {
+		Files []string `json:"files"`
+	}
+	body, _ := io.ReadAll(w.Body)
+	assert.NoError(t, json.Unmarshal(body, &response))
+	
+	// Verify all test files are returned
+	assert.Len(t, response.Files, 3)
+	assert.Contains(t, response.Files, "file1.md")
+	assert.Contains(t, response.Files, "file2.md")
+	assert.Contains(t, response.Files, "file3.md")
+}
+
+func TestFileListingWithNonMarkdownFiles(t *testing.T) {
+	dataDir, err := setupTestDir()
+	if err != nil {
+		t.Fatalf("Failed to create test dir: %v", err)
+	}
+	defer cleanupTestDir(dataDir)
+
+	// Create test files including non-markdown files
+	testFiles := []string{
+		"file1.md",
+		"file2.txt",
+		"file3.md",
+		"file4.log",
+	}
+	
+	for _, filename := range testFiles {
+		filepath := filepath.Join(dataDir, filename)
+		content := "Content of " + filename
+		if err := os.WriteFile(filepath, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to create test file %s: %v", filename, err)
+		}
+	}
+
+	log := logger.NewLogger()
+	handler := api.NewAPIHandler(dataDir, log)
+	router := mux.NewRouter()
+	router.Handle("/api", handler).Methods("GET")
+	router.Handle("/api/{filename:.+\\.md}", handler)
+
+	// Test GET /api (list files)
+	req := httptest.NewRequest("GET", "/api", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+	assert.Equal(t, http.StatusOK, w.Code)
+	
+	var response struct {
+		Files []string `json:"files"`
+	}
+	body, _ := io.ReadAll(w.Body)
+	assert.NoError(t, json.Unmarshal(body, &response))
+	
+	// Verify only markdown files are returned
+	assert.Len(t, response.Files, 2)
+	assert.Contains(t, response.Files, "file1.md")
+	assert.Contains(t, response.Files, "file3.md")
+	assert.NotContains(t, response.Files, "file2.txt")
+	assert.NotContains(t, response.Files, "file4.log")
+}
+
+func TestFileListingEmptyDirectory(t *testing.T) {
+	dataDir, err := setupTestDir()
+	if err != nil {
+		t.Fatalf("Failed to create test dir: %v", err)
+	}
+	defer cleanupTestDir(dataDir)
+
+	log := logger.NewLogger()
+	handler := api.NewAPIHandler(dataDir, log)
+	router := mux.NewRouter()
+	router.Handle("/api", handler).Methods("GET")
+	router.Handle("/api/{filename:.+\\.md}", handler)
+
+	// Test GET /api (list files in empty directory)
+	req := httptest.NewRequest("GET", "/api", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+	assert.Equal(t, http.StatusOK, w.Code)
+	
+	var response struct {
+		Files []string `json:"files"`
+	}
+	body, _ := io.ReadAll(w.Body)
+	assert.NoError(t, json.Unmarshal(body, &response))
+	
+	// Verify empty array is returned
+	assert.Len(t, response.Files, 0)
+}