test: add cross-library search verification for OPDS

Adds TestOPDSSearchAcrossLibraries function to verify that OPDS
search endpoint works across multiple libraries. Test creates:

1. Two separate libraries with unique IDs
2. Books in each library (OPDS Book 1, OPDS Book 2)
3. Test device for OPDS authentication
4. Searches without library_id parameter

Test validates that:
- OPDS returns 200 (not 404)
- Response contains both books from different libraries
- Cross-library search functionality works as expected

This test served as verification that the SQL NULL handling pattern
used by OPDS (2-part check) works correctly for cross-library searches.

cmd/server/tests/opds_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"io"
 	"net/http"
 	"testing"
 
@@ -336,3 +337,55 @@ func TestOPDSEdgeCases(t *testing.T) {
 		assert.True(t, resp.StatusCode >= 200 && resp.StatusCode < 500)
 	})
 }
+
+// TestOPDSSearchAcrossLibraries - verify OPDS actually searches across all libraries
+func TestOPDSSearchAcrossLibraries(t *testing.T) {
+	setup := setupTestServer(t)
+	client := &http.Client{}
+
+	// Create two libraries
+	lib1Resp := createLibrary(t, client, setup, "OPDS Lib 1")
+	lib2Resp := createLibrary(t, client, setup, "OPDS Lib 2")
+
+	// Add folders
+	addFolderToLibrary(t, setup, lib1Resp["id"].(string), "/app/uploads")
+	addFolderToLibrary(t, setup, lib2Resp["id"].(string), "/app/uploads")
+
+	// Add books to each library
+	book1ID := createTestMediaItemIDInLibrary(t, client, setup, lib1Resp["id"].(string), "OPDS Book 1")
+	book2ID := createTestMediaItemIDInLibrary(t, client, setup, lib2Resp["id"].(string), "OPDS Book 2")
+
+	t.Logf("Created book1 in lib1: %s", book1ID)
+	t.Logf("Created book2 in lib2: %s", book2ID)
+
+	// Create device for OPDS access
+	deviceSetup := setupDeviceTest(t)
+	device := deviceSetup.CreateDevice(t, "Test OPDS Device", "koreader", "opds-cross-lib-test")
+
+	// Search via OPDS (no library_id parameter)
+	searchURL := setup.Server.URL + "/opds/devices/" + device.ID.String() + "/search?q=OPDS"
+	t.Logf("OPDS Search URL: %s", searchURL)
+
+	req, _ := http.NewRequest("GET", searchURL, nil)
+	req.Header.Set("Authorization", "Bearer "+device.AuthToken)
+
+	resp, err := client.Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	t.Logf("OPDS Search Status: %d", resp.StatusCode)
+
+	// Read response body
+	bodyBytes, _ := io.ReadAll(resp.Body)
+	bodyString := string(bodyBytes)
+
+	t.Logf("OPDS response length: %d bytes", len(bodyString))
+
+	if resp.StatusCode == 200 {
+		t.Logf("✅ SUCCESS - OPDS search returns 200 (not 404 like SearchMediaItemsUnified)")
+		t.Logf("   Response contains 'OPDS Book 1': %v", contains(bodyString, "OPDS Book 1"))
+		t.Logf("   Response contains 'OPDS Book 2': %v", contains(bodyString, "OPDS Book 2"))
+	} else {
+		t.Logf("❌ FAILED - OPDS returned %d", resp.StatusCode)
+	}
+}