enhance: Migrate remaining gopkg.in/yaml.v3 usages to go.yaml.in/yaml/v4 (#37866)

### Description
Replaces all remaining direct `gopkg.in/yaml.v3` imports with
`go.yaml.in/yaml/v4` across models, modules, routers, services, and
integration tests. `gopkg.in/yaml.v3` moves from a direct to an indirect
dependency in `go.mod`.

#### API compatibility

The yaml.Node type, node.Kind/node.Content traversal style
(modules/markup/markdown/convertyaml.go), and the
UnmarshalYAML(*yaml.Node) interface signature
(modules/optional/serialization.go) are all preserved in v4 — no
call-site changes were required beyond the import path.

**Related:**
- https://github.com/go-gitea/gitea/pull/36564#issuecomment-4526536805

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

routers/api/packages/helm/helm.go

@@ -23,7 +23,7 @@ import (
 	"gitea.dev/services/context"
 	packages_service "gitea.dev/services/packages"
 
-	"gopkg.in/yaml.v3"
+	"go.yaml.in/yaml/v4"
 )
 
 func apiError(ctx *context.Context, status int, obj any) {

routers/web/devtest/mail_preview.go

@@ -12,7 +12,7 @@ import (
 	"gitea.dev/services/context"
 	"gitea.dev/services/mailer"
 
-	"gopkg.in/yaml.v3"
+	"go.yaml.in/yaml/v4"
 )
 
 func MailPreviewRender(ctx *context.Context) {