fix: enforce public-only token scope and harden push options / locale parsing (#38323) (#38399)

Backport #38323 by @bircni

Three independent security hardening fixes, each with a regression test:

- **Locale DoS:** the `Locale` middleware passed the raw
`Accept-Language` header to `ParseAcceptLanguage`, whose guard only
counts `-` while the scanner aliases `_` to `-` — a large `_`-separated
header on an unauthenticated request burned CPU. The header is now
length-bounded before parsing.
- **Public-only token scope:** `GET /teams/{id}/repos`,
`.../repos/{org}/{repo}`, `/teams/{id}/activities/feeds`, and
`/users/{username}/orgs/{org}/permissions` still returned private
repo/activity/permission data to a public-only token. They now filter
via `TokenCanAccessRepo` / `ApplyPublicOnly` and reject non-public org
permissions.
- **Push-option visibility:** `repo.private` / `repo.template` push
options were applied to any existing repo, letting an owner/admin
silently flip visibility bypassing audit, webhooks, and notifications.
They are now honored only on push-to-create.

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

routers/api/v1/org/org.go

@@ -145,6 +145,13 @@ func GetUserOrgsPermissions(ctx *context.APIContext) {
 
 	op := api.OrganizationPermissions{}
 
+	// A public-only token must not disclose membership/permission details of a
+	// non-public org, even for the token owner's own private orgs.
+	if ctx.PublicOnly && !o.Visibility.IsPublic() {
+		ctx.APIErrorNotFound()
+		return
+	}
+
 	if !organization.HasOrgOrUserVisible(ctx, o, ctx.Doer) {
 		ctx.APIErrorNotFound()
 		return