feat(middleware): add customizable error handler for auth failures

[umputun]

Adds ErrorHandlerFunc type to customize error responses on auth failures. Related to #264

What changed:

• new ErrorHandlerFunc type in middleware package
• ErrorHandler field in Authenticator struct and Opts
• all middleware (Auth, AdminOnly, RBAC) now use errResponse which calls custom handler if set
• default behavior unchanged (returns "Unauthorized" 401 or "Access denied" 403)

Usage example:

options := auth.Opts{
    ErrorHandler: middleware.ErrorHandlerFunc(func(w http.ResponseWriter, r *http.Request, code int, err error) {
        if strings.HasPrefix(r.URL.Path, "/api/") {
            w.Header().Set("Content-Type", "application/json")
            w.WriteHeader(code)
            json.NewEncoder(w).Encode(map[string]string{"error": "authentication required"})
            return
        }
        http.Redirect(w, r, "/login", http.StatusFound)
    }),
}

@fbuedding pls take a look and let me know if this covers the functionality you had in mind

[Copilot]

Pull request overview

This PR adds customizable error handling for authentication failures in the middleware package, addressing issue #264. The implementation allows users to customize how authentication errors are handled (e.g., returning JSON responses for API routes or redirecting to login pages for web routes) while maintaining backward compatibility with the default behavior.

Changes:

• Added ErrorHandlerFunc type to allow custom error response handling on authentication failures
• Extended Authenticator struct and Opts to include optional ErrorHandler field
• Refactored error responses in Auth, AdminOnly, and RBAC middlewares to use a centralized errResponse method

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
v2/middleware/auth.go	Added ErrorHandlerFunc type, ErrorHandler field to Authenticator, and errResponse helper method; updated Auth, AdminOnly, and RBAC to use custom error handler
v2/auth.go	Added ErrorHandler field to Opts and wired it to Authenticator in NewService
v2/middleware/auth_test.go	Added comprehensive test coverage for custom error handlers in Auth, AdminOnly, and RBAC middlewares
README.md	Added documentation and usage example for the new ErrorHandlerFunc feature
.gitignore	Added entries for ralph progress logs (unrelated to this PR)

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[coveralls]

Pull Request Test Coverage Report for Build 21151713439

Details

• 14 of 16 (87.5%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 83.628%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
v2/middleware/auth.go	13	15	86.67%

Totals
Change from base Build 20442464979:	0.05%
Covered Lines:	2697
Relevant Lines:	3225

---

💛 - Coveralls

[fbuedding]

This is exactly what I had in mind :-) Thank you!

[umputun]

oops, just noticed in your original ticket: "If you're open to it, I could try to implement these
changes myself and create a pull request."

sorry, missed it completely and jumped on this one ;)

[fbuedding]

oops, just noticed in your original ticket: "If you're open to it, I could try to implement these changes myself and create a pull request."

sorry, missed it completely and jumped on this one ;)

No worries! I'm always eager to contribute :D. When I saw that function literal, I thought it would be great to make it customizable and didn't want to 'demand' a change that I could implement myself.