feat(router): enhance routing capabilities with new features

[KARTIKrocks]

• Added support for custom 404 Not Found and 405 Method Not Allowed handlers via WithNotFound and WithMethodNotAllowed options.
• Introduced WithStripSlash and WithRedirectSlash options for handling trailing slashes in routes.
• Implemented URL method for generating URLs for named routes with parameter substitution.
• Added Route method for inline sub-routing, allowing nested route definitions.
• Enhanced RouteEntry to support named routes and improved middleware handling with With method.
• Introduced Walk method to iterate over registered routes and Routes method to retrieve a snapshot of all routes.
• Updated error handling in DefaultErrorHandler to ensure proper response formatting.
• Added tests for new features, including middleware application, route handling, and error scenarios.

Summary

Added

• router — Named routes via RouteEntry.Name(name) and Router.URL(name, params...) for reverse URL generation with {param} and {param...} placeholder substitution
• router — Route introspection: Router.Routes() returns a snapshot of all registered RouteInfo entries; Router.Walk(fn) iterates with early-exit support
• router — RouteInfo.HandlerName captures the original handler's function name via runtime.FuncForPC for debugging and documentation
• router — Parameter constraints: ValidateParams(handler, constraints...) wraps a handler with path-parameter validation; built-in constraint constructors Int, UUID, Regex, OneOf
• router — With(middleware...) returns a child group for per-route inline middleware without affecting sibling routes
• router — Route(prefix, fn, middleware...) for inline sub-routing — creates a child group, calls fn to register routes, and returns the group for further use
• router — Mount(prefix, handler) attaches an http.Handler (or *Router) at a prefix with http.StripPrefix; sub-router routes and named routes are merged into the parent's route table
• router — Static(prefix, dir) serves files from a filesystem directory; File(pattern, filePath) serves a single file for GET requests
• router — WithNotFound(handler) and WithMethodNotAllowed(handler) options for custom 404/405 handlers, taking precedence over the ErrorHandler
• router — WithStripSlash() silently removes trailing slashes before routing; WithRedirectSlash() sends 301 redirects (mutually exclusive, panics if both set)

Changed

• router — All method helpers (Get, Post, Put, Patch, Delete, Head, Options and their Func variants) and Handle/HandleFunc now return *RouteEntry for optional .Name() chaining
• router — register() accepts an additional origFn parameter to capture the original handler name before middleware wrapping

Checklist

• fmt, vet, lint, test, build passes (make all)
• New code has tests where appropriate
• Breaking changes are documented