Keeping consistent between UI and API about combined commit status state and fix some bugs (#34562)

Extract from #34531 

## Move Commit status state to a standalone package

Move the state from `structs` to `commitstatus` package. It also
introduce `CommitStatusStates` so that the combine function could be
used from UI and API logic.

## Combined commit status Changed

This PR will follow Github's combined commit status. Before this PR,
every commit status could be a combined one.
According to
https://docs.github.com/en/rest/commits/statuses?apiVersion=2022-11-28#get-the-combined-status-for-a-specific-reference
> Additionally, a combined state is returned. The state is one of:
> failure if any of the contexts report as error or failure
> pending if there are no statuses or a context is pending
> success if the latest status for all contexts is success

This PR will follow that rule and remove the `NoBetterThan` logic. This
also fixes the inconsistent between UI and API. In the API convert
package, it has implemented this which is different from the UI. It also
fixed the missing `URL` and `CommitURL` in the API.

## `CalcCommitStatus` return nil if there is no commit statuses

The behavior of `CalcCommitStatus` is changed. If the parameter commit
statuses is empty, it will return nil. The reference places should check
the returned value themselves.

tests/integration/pull_status_test.go

@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/models/issues"
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/commitstatus"
 	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/test"
@@ -55,20 +56,20 @@ func TestPullCreate_CommitStatus(t *testing.T) {
 
 		commitID := path.Base(commitURL)
 
-		statusList := []api.CommitStatusState{
-			api.CommitStatusPending,
-			api.CommitStatusError,
-			api.CommitStatusFailure,
-			api.CommitStatusSuccess,
-			api.CommitStatusWarning,
+		statusList := []commitstatus.CommitStatusState{
+			commitstatus.CommitStatusPending,
+			commitstatus.CommitStatusError,
+			commitstatus.CommitStatusFailure,
+			commitstatus.CommitStatusSuccess,
+			commitstatus.CommitStatusWarning,
 		}
 
-		statesIcons := map[api.CommitStatusState]string{
-			api.CommitStatusPending: "octicon-dot-fill",
-			api.CommitStatusSuccess: "octicon-check",
-			api.CommitStatusError:   "gitea-exclamation",
-			api.CommitStatusFailure: "octicon-x",
-			api.CommitStatusWarning: "gitea-exclamation",
+		statesIcons := map[commitstatus.CommitStatusState]string{
+			commitstatus.CommitStatusPending: "octicon-dot-fill",
+			commitstatus.CommitStatusSuccess: "octicon-check",
+			commitstatus.CommitStatusError:   "gitea-exclamation",
+			commitstatus.CommitStatusFailure: "octicon-x",
+			commitstatus.CommitStatusWarning: "gitea-exclamation",
 		}
 
 		testCtx := NewAPITestContext(t, "user1", "repo1", auth_model.AccessTokenScopeWriteRepository)
@@ -99,7 +100,7 @@ func TestPullCreate_CommitStatus(t *testing.T) {
 
 		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerName: "user1", Name: "repo1"})
 		css := unittest.AssertExistsAndLoadBean(t, &git_model.CommitStatusSummary{RepoID: repo1.ID, SHA: commitID})
-		assert.Equal(t, api.CommitStatusWarning, css.State)
+		assert.Equal(t, commitstatus.CommitStatusSuccess, css.State)
 	})
 }