fix(model): update api tests to use JSON instead of msgpack

Update test assertions to verify JSON content type instead of msgpack,
aligning with the recent change to use JSON as default HTTP send method.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: AnnatarHe

model/api_test.go

@@ -2,14 +2,14 @@ package model
 
 import (
 	"context"
+	"encoding/json"
 	"net/http"
 	"net/http/httptest"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/suite"
-	"github.com/vmihailenco/msgpack/v5"
 )
 
 type apiTestSuite struct {
@@ -21,13 +21,13 @@ func (s *apiTestSuite) TestDoSendData() {
 	s.T().Run("successful request", func(t *testing.T) {
 		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			// Verify headers
-			assert.Equal(t, "application/msgpack", r.Header.Get("Content-Type"))
+			assert.Equal(t, "application/json", r.Header.Get("Content-Type"))
 			assert.Contains(t, r.Header.Get("User-Agent"), "shelltimeCLI@")
 			assert.Equal(t, "CLI testToken", r.Header.Get("Authorization"))
 
 			// Decode request body
 			var payload PostTrackArgs
-			err := msgpack.NewDecoder(r.Body).Decode(&payload)
+			err := json.NewDecoder(r.Body).Decode(&payload)
 			assert.NoError(t, err)
 
 			// Verify payload