feat(group): add SetNotFoundHandle support for router groups

[devfeel]

Summary

• Add SetNotFoundHandle method to Group interface
• Add notFoundHandler field to xGroup struct
• Group-level NotFoundHandler takes priority over app-level handler
• Fix prefix matching to avoid false positives (e.g., /api_v2 matching /api)
• Add unit tests for SetNotFoundHandle functionality
• Add example code in example/group/

Usage Example

// Create API group
apiGroup := server.Group("/api")

// Set group-level 404 handler
apiGroup.SetNotFoundHandle(func(ctx dotweb.Context) {
    ctx.Response().Header().Set("Content-Type", "application/json")
    ctx.WriteString(`{"code": 404, "message": "API resource not found"}`)
})

// Register routes
apiGroup.GET("/users", func(ctx dotweb.Context) {
    ctx.WriteString("Users list")
})

Behavior

• Request to /api/users → Returns "Users list"
• Request to /api/unknown → Returns JSON 404 response (group handler)
• Request to /api_v2/test → Returns global 404 (not matched by /api group)
• Request to /unknown → Returns global 404 (app handler)

Test Plan

• All unit tests pass
• Prefix matching fix verified
• Example code compiles

Resolves: #229