feat: add SetNotFoundHandler and SetMethodNotAllowedHandler for custom 404 and 405 responses

[qm012]

related: #38

Adds two setters to App for customizing error responses:

• SetNotFoundHandler — served when a request matches no registered pattern (404)
• SetMethodNotAllowedHandler — served when a path matches a pattern but its method does not (405)

Both are optional: leaving them unset keeps the mux's default responses, and
ServeHTTP takes a fast path straight to the mux in that case.

Why

http.ServeMux has no public hook for its 404/405 handlers (see
golang/go#65648), so apps could not customize
error pages or JSON error bodies without bypassing the mux entirely.

Usage

JSON API errors — return a structured body instead of the mux's plain text:

app := sim.NewApp()
app.Get("/users/{id}", userHandler)

app.SetNotFoundHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(http.StatusNotFound)
    _, _ = io.WriteString(w, `{"code":404,"message":"resource not found"}`)
}))

app.SetMethodNotAllowedHandler(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    // Allow is already set to the methods the mux computed; keep it.
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(http.StatusMethodNotAllowed)
    _, _ = io.WriteString(w, `{"code":405,"message":"method not allowed"}`)
}))

_ = app.Run(context.Background(), ":8080")

A single handler can cover both cases if only the status differs:

errorPage := func(code int) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
        w.Header().Set("Content-Type", "text/html; charset=utf-8")
        w.WriteHeader(code)
        fmt.Fprintf(w, "<h1>%d</h1><p><a href=\"/\">Back home</a></p>", code)
    })
}
app.SetNotFoundHandler(errorPage(http.StatusNotFound))
app.SetMethodNotAllowedHandler(errorPage(http.StatusMethodNotAllowed))

Notes:

• Handlers set on the root app are inherited by groups created with App.Group, so sub-routers share the same error pages.
• Setting only one handler leaves the other on the mux default.
• The 405 handler receives the response with Allow pre-populated per RFC 9110; it may override or delete it.

[qm012]

With custom error handlers set, matched requests now match twice (mux.Handler + mux.ServeHTTP), since Handler doesn't populate r.Pattern/PathValue. I assume that's acceptable as it's slow-path only — the alternative (a WriteHeader-intercepting wrapper) looks like more complexity than it saves. Worth confirming.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/qm012/sim	90.51% (+1.09%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/qm012/sim/app.go	76.60% (+8.95%)	94 (+26)	72 (+26)	22	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/qm012/sim/app_test.go