fix(repo): commit page fails to render unsigned commits with a different committer (#39381)

Since #39229 the commit page header dereferences
`.Verification.CommittingUser` when the committer is not the author.
`Verification` is `nil` for unsigned commits (see `repo.Diff`), so
opening such a commit — a rebased or cherry-picked one, for example —
logs a template error and the page comes out truncated:

```
Render failed: failed to render template: repo/commit_page, error: template error: builtin(bindata):repo/commit_page:138:22 : executing "repo/commit_page" at <.Verification.CommittingUser>: nil pointer evaluating interface {}.CommittingUser
```

This guards the access and adds an integration test that creates a
commit with distinct author and committer identities and checks the page
renders completely (the status stays 200 on a mid-render failure, so the
test looks at the body).

_The fix was worked out with help from an AI assistant; I reviewed and
tested it myself._

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/common/errpage.go

@@ -21,7 +21,11 @@ import (
 	"gitea.dev/services/context"
 )
 
-const tplStatus500 templates.TplName = "status/500"
+const (
+	tplStatus500 templates.TplName = "status/500"
+
+	PageInternalServerErrorMark = "status-page-500"
+)
 
 func renderServerErrorPage(w http.ResponseWriter, req *http.Request, respCode int, tmpl templates.TplName, ctxData map[string]any, plainMsg string) {
 	acceptsHTML := false