fix(router): keep group auto-404 in sync with root RouteNotFound

[Solaris-star]

Summary

When a group has middleware, Echo auto-registers RouteNotFound routes with a nil handler so the group middleware still runs on 404s. Those handlers are filled from DefaultRouter.notFoundHandler.

e.RouteNotFound("/*", h) only installed h on the root tree node. It did not update r.notFoundHandler, so later group auto-404 routes kept the default JSON {"message":"Not Found"} instead of the root catch-all.

Fix

On DefaultRouter.Add, if method is RouteNotFound and path is /* or "", also set r.notFoundHandler = route.Handler.

Test plan

• TestGroup_RouteNotFoundFallsBackToRoot (issue repro: /foo, /v0/foo, /v1/foo)
• TestGroup_RouteNotFound* / TestGroup_UseMultipleTimes
• go test .

Fixes #2485

[aldas]

I do not think adding a route should have side-effects like that to Router.

you can achieve same (global 404 handler) when you create Router with your own NotFoundHandler

so this

	e := echo.New()
	e.RouteNotFound("/*", func(c *echo.Context) error {
		return c.NoContent(http.StatusNotFound)
	})

should be

	e := echo.NewWithConfig(echo.Config{
		Router: echo.NewRouter(echo.RouterConfig{
			NotFoundHandler: func(c *echo.Context) error {
				return c.NoContent(http.StatusNotFound)
			},
		}),
	})