fix: propagate root RouteNotFound handler to route groups

[mayank-dev-15]

Summary

When a group has middleware, Group.Use() registers catch-all routes ("" and "/*") to guarantee middleware execution. Previously these routes always used the hardcoded NotFoundHandler, ignoring any custom RouteNotFound handler set on the root Echo instance.

Problem

e := echo.New()
e.RouteNotFound("/*", customHandler) // user's custom 404

v0 := e.Group("/v0", someMiddleware)
v0.POST("/resource", handler)

// Request to /v0/nonexistent uses NotFoundHandler instead of customHandler

Fix

Store the user's custom RouteNotFound handler on the Echo struct and use it when registering group catch-all routes, falling back to NotFoundHandler if none was set.

Changes

• echo.go: Added notFoundHandler field to Echo struct; set it in RouteNotFound()
• group.go: Use() now uses g.echo.notFoundHandler instead of hardcoded NotFoundHandler
• group_test.go: Updated test expectations to verify root handler propagation

Fixes #2485

[aldas]

you can already do this by

	custom404Handler := func(c echo.Context) error {
		return c.String(http.StatusNotFound, "Custom 404 message")
	}
	echo.NotFoundHandler = custom404Handler

	e.RouteNotFound("/*", custom404Handler) // user's custom 404

and groups will use that same customHandler as it echo.NotFoundHandler