refactor(types): unify handler and middleware definitions

- Consolidates `Handler` and `Middleware` types under `Types` module
- Replaces `IHandler` and `IMiddleware` interfaces with typed functions
- Simplifies imports and improves code organization
- Enhances debugging with named handlers and middlewares

src/Types/Handler.ts

@@ -0,0 +1,57 @@
+import { IContext } from '../Interfaces/mod.ts';
+
+/**
+ * Represents a final request handler responsible for producing an HTTP response.
+ *
+ * The handler is the terminal stage of the middleware pipeline and is responsible
+ * for processing the incoming request and generating the final `Response`.
+ *
+ * It receives the fully-typed request context, which includes the original request,
+ * parsed route parameters, query parameters, and any shared state populated by prior middleware.
+ *
+ * @template TContext The specific context type for this handler, including typed `state`, `params`, and `query`.
+ */
+type Handler<TContext extends IContext = IContext> = (
+    ctx: TContext,
+) => Promise<Response>;
+
+/**
+ * Represents a handler function with an associated name.
+ *
+ * This is useful for debugging, logging, or when you need to reference
+ * the handler by name in your application.
+ *
+ * @template TContext The specific context type for this handler, including typed `state`, `params`, and `query`.
+ */
+type NamedHandler<TContext extends IContext = IContext> =
+    & Handler<TContext>
+    & { name?: string };
+
+export type { NamedHandler as Handler };
+
+/**
+ * Type guard to determine whether a given value is a valid `IHandler` function.
+ *
+ * This function checks whether the input is a function and whether it returns
+ * a `Promise<Response>` when called. Due to TypeScript's structural typing and
+ * the lack of runtime type information, only minimal runtime validation is possible.
+ *
+ * @param value - The value to test.
+ * @returns `true` if the value is a function that appears to conform to `IHandler`.
+ *
+ * @example
+ * ```ts
+ * const candidate = async (ctx: IContext) => new Response("ok");
+ * if (isHandler(candidate)) {
+ *   // candidate is now typed as IHandler<IContext>
+ * }
+ * ```
+ */
+export function isHandler<TContext extends IContext = IContext>(
+    value: unknown,
+): value is Handler<TContext> {
+    return (
+        typeof value === 'function' &&
+        value.length === 1 // ctx
+    );
+}