test(context): add comprehensive unit tests for Context.File() method (#4307)

* test: add comprehensive unit tests for Context.File() method

- Add TestContextFile with multiple test scenarios in context_test.go
- Add simplified tests in context_file_test.go
- Cover file serving, 404 handling, directory access, HEAD requests, and Range requests
- All tests pass successfully

* fix: resolve gci formatting issues in test files

* fix: correct test file paths and add test file to gin directory

* move test_file.txt to testdata directory as suggested by maintainer

* fix: update test file paths to use testdata directory

context_file_test.go

@@ -0,0 +1,35 @@
+package gin
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestContextFileSimple tests the Context.File() method with a simple case
+func TestContextFileSimple(t *testing.T) {
+	// Test serving an existing file
+	testFile := "testdata/test_file.txt"
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/test", nil)
+
+	c.File(testFile)
+
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Contains(t, w.Body.String(), "This is a test file")
+	assert.Equal(t, "text/plain; charset=utf-8", w.Header().Get("Content-Type"))
+}
+
+// TestContextFileNotFound tests serving a non-existent file
+func TestContextFileNotFound(t *testing.T) {
+	w := httptest.NewRecorder()
+	c, _ := CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/test", nil)
+
+	c.File("non_existent_file.txt")
+
+	assert.Equal(t, http.StatusNotFound, w.Code)
+}