fix(api): nil pointer panic when filtering tracked times by a non-existent user (#38112)

## Problem

`GET /repos/{owner}/{repo}/times` and `GET
/repos/{owner}/{repo}/issues/{index}/times` crash with a nil pointer
dereference when the `user` query filter names a user that does not
exist.

## Root cause

In `ListTrackedTimes` and `ListTrackedTimesByRepository`, the
`IsErrUserNotExist` branch sends the 404 but is missing a `return`, so
execution falls through to `opts.UserID = user.ID` with a nil `user`.

---------

Co-authored-by: bircni <bircni@icloud.com>

routers/api/v1/repo/issue_tracked_time.go

@@ -91,6 +91,7 @@ func ListTrackedTimes(ctx *context.APIContext) {
 		user, err := user_model.GetUserByName(ctx, qUser)
 		if user_model.IsErrUserNotExist(err) {
 			ctx.APIError(http.StatusNotFound, err.Error())
+			return
 		} else if err != nil {
 			ctx.APIErrorInternal(err)
 			return
@@ -499,6 +500,7 @@ func ListTrackedTimesByRepository(ctx *context.APIContext) {
 		user, err := user_model.GetUserByName(ctx, qUser)
 		if user_model.IsErrUserNotExist(err) {
 			ctx.APIError(http.StatusNotFound, err.Error())
+			return
 		} else if err != nil {
 			ctx.APIErrorInternal(err)
 			return

tests/integration/api_issue_tracked_time_test.go

@@ -13,6 +13,7 @@ import (
 	issues_model "gitea.dev/models/issues"
 	"gitea.dev/models/unittest"
 	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/json"
 	api "gitea.dev/modules/structs"
 	"gitea.dev/tests"
 
@@ -61,6 +62,44 @@ func TestAPIGetTrackedTimes(t *testing.T) {
 	assert.Equal(t, int64(6), filterAPITimes[1].ID)
 }
 
+// TestAPIGetTrackedTimesNonExistentUserFilter ensures filtering by a user that
+// does not exist returns a clean 404 instead of panicking (nil pointer dereference).
+func TestAPIGetTrackedTimesNonExistentUserFilter(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	issue2 := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 2})
+	assert.NoError(t, issue2.LoadRepo(t.Context()))
+
+	session := loginUser(t, user2.Name)
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeReadIssue, auth_model.AccessTokenScopeReadRepository)
+
+	for _, tc := range []struct {
+		name string
+		url  string
+	}{
+		{"repository level", fmt.Sprintf("/api/v1/repos/%s/%s/times?user=nonexistentuser", user2.Name, issue2.Repo.Name)},
+		{"issue level", fmt.Sprintf("/api/v1/repos/%s/%s/issues/%d/times?user=nonexistentuser", user2.Name, issue2.Repo.Name, issue2.Index)},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			req := NewRequest(t, "GET", tc.url).AddTokenAuth(token)
+			resp := MakeRequest(t, req, http.StatusNotFound)
+
+			assert.True(t, json.Valid(resp.Body.Bytes()), "response body must be a single JSON value, got: %s", resp.Body.Bytes())
+
+			var apiError api.APIError
+			DecodeJSON(t, resp, &apiError)
+			assert.Contains(t, apiError.Message, "user does not exist")
+		})
+	}
+
+	t.Run("existing user", func(t *testing.T) {
+		req := NewRequestf(t, "GET", "/api/v1/repos/%s/%s/times?user=%s", user2.Name, issue2.Repo.Name, user2.Name).AddTokenAuth(token)
+		resp := MakeRequest(t, req, http.StatusOK)
+		DecodeJSON(t, resp, api.TrackedTimeList{})
+	})
+}
+
 func TestAPIDeleteTrackedTime(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()