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)
			},
		}),
	})

[Solaris-star]

@aldas agreed — a call to add a route shouldn't mutate the Router like that. I've dropped the side-effect entirely.

What the reworked PR does instead:

• Reverts the r.notFoundHandler mutation in DefaultRouter.Add.
• Documents that the supported way to customise the 404 for groups (which auto-register catch-all routes when they have middleware) is RouterConfig.NotFoundHandler, exactly as you pointed out — the group's nil-handler 404 routes resolve to it at registration time, and group middleware still wraps them.
• Updates the two TestGroup_RouteNotFoundWithMiddleware cases that had encoded the old (side-effecting) expectation: they now assert the group auto-404 uses the configured NotFoundHandler (global), not a later e.RouteNotFound.
• Adds TestGroup_RouteNotFoundUsesRouterConfig demonstrating the recommended pattern end-to-end.

So this PR now just documents + tests the existing intended behaviour rather than changing routing. If you think #2485 is better served purely by a docs note and no code/test changes at all, I can trim it further — let me know.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.34%. Comparing base (dcb05f0) to head (51adb32).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3052   +/-   ##
=======================================
  Coverage   93.34%   93.34%           
=======================================
  Files          43       43           
  Lines        4735     4735           
=======================================
  Hits         4420     4420           
  Misses        192      192           
  Partials      123      123           

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[aldas]

LGTM