Remove IsValidExternalURL/IsAPIURL and use IsValidURL at call sites (#37364)

This PR simplifies URL validation by removing `IsValidExternalURL` and
`IsAPIURL` from `modules/validation/helpers.go` and switching repository
settings/API callers to `IsValidURL`.
It also aligns tracker-format validation and tests with the new helper
surface.

- **Validation helpers**
  - Removed `IsValidExternalURL` and `IsAPIURL`.
  - Updated `IsValidExternalTrackerURLFormat` to depend on `IsValidURL`.

- **Caller updates**
- Replaced `validation.IsValidExternalURL(...)` with
`validation.IsValidURL(...)` in:
    - `routers/web/repo/setting/setting.go`
    - `routers/api/v1/repo/repo.go`

- **Tests**
  - Removed tests dedicated to `IsValidExternalURL`.
- Updated tracker-format test expectations to match `IsValidURL`-based
behavior.

```go
// before
if !validation.IsValidExternalURL(form.ExternalTrackerURL) { ... }

// after
if !validation.IsValidURL(form.ExternalTrackerURL) { ... }
```

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>

modules/validation/helpers_test.go

@@ -6,9 +6,6 @@ package validation
 import (
 	"testing"
 
-	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/test"
-
 	"github.com/stretchr/testify/assert"
 )
 
@@ -47,51 +44,7 @@ func Test_IsValidURL(t *testing.T) {
 	}
 }
 
-func Test_IsValidExternalURL(t *testing.T) {
-	defer test.MockVariableValue(&setting.AppURL, "https://try.gitea.io/")()
-
-	cases := []struct {
-		description string
-		url         string
-		valid       bool
-	}{
-		{
-			description: "Current instance URL",
-			url:         "https://try.gitea.io/test",
-			valid:       true,
-		},
-		{
-			description: "Loopback IPv4 URL",
-			url:         "http://127.0.1.1:5678/",
-			valid:       false,
-		},
-		{
-			description: "Current instance API URL",
-			url:         "https://try.gitea.io/api/v1/user/follow",
-			valid:       false,
-		},
-		{
-			description: "Local network URL",
-			url:         "http://192.168.1.2/api/v1/user/follow",
-			valid:       true,
-		},
-		{
-			description: "Local URL",
-			url:         "http://LOCALHOST:1234/whatever",
-			valid:       false,
-		},
-	}
-
-	for _, testCase := range cases {
-		t.Run(testCase.description, func(t *testing.T) {
-			assert.Equal(t, testCase.valid, IsValidExternalURL(testCase.url))
-		})
-	}
-}
-
 func Test_IsValidExternalTrackerURLFormat(t *testing.T) {
-	defer test.MockVariableValue(&setting.AppURL, "https://try.gitea.io/")()
-
 	cases := []struct {
 		description string
 		url         string
@@ -105,7 +58,7 @@ func Test_IsValidExternalTrackerURLFormat(t *testing.T) {
 		{
 			description: "Local external tracker URL with all placeholders",
 			url:         "https://127.0.0.1/{user}/{repo}/issues/{index}",
-			valid:       false,
+			valid:       true,
 		},
 		{
 			description: "External tracker URL with typo placeholder",