docs(httputil): correct tier from 3 to 2

httputil depends on xerrors (Tier 0) and valid (Tier 1), placing it at
Tier 2. No infrastructure or lifecycle dependencies exist in this module.

handle.go

@@ -0,0 +1,86 @@
+package httputil
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+
+	"code.nochebuena.dev/go/valid"
+	"code.nochebuena.dev/go/xerrors"
+)
+
+// HandlerFunc is an http.Handler that can return an error.
+// On non-nil error the error is mapped to the appropriate HTTP response via Error.
+// Useful for manual handlers that don't need the typed adapter.
+type HandlerFunc func(w http.ResponseWriter, r *http.Request) error
+
+func (h HandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if err := h(w, r); err != nil {
+		Error(w, err)
+	}
+}
+
+// Handle adapts a typed business function to http.HandlerFunc.
+// - Decodes JSON request body into Req.
+// - Validates Req using the provided Validator.
+// - Calls fn with the request context and decoded Req.
+// - Encodes Res as JSON with HTTP 200.
+// - Maps any returned error to the appropriate HTTP status via xerrors code.
+func Handle[Req, Res any](v valid.Validator, fn func(ctx context.Context, req Req) (Res, error)) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		var req Req
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			Error(w, badRequest("invalid JSON: "+err.Error()))
+			return
+		}
+		if err := v.Struct(req); err != nil {
+			Error(w, err)
+			return
+		}
+		res, err := fn(r.Context(), req)
+		if err != nil {
+			Error(w, err)
+			return
+		}
+		JSON(w, http.StatusOK, res)
+	}
+}
+
+// HandleNoBody adapts a typed function with no request body (GET, DELETE).
+// Calls fn with request context; encodes result as JSON with HTTP 200.
+func HandleNoBody[Res any](fn func(ctx context.Context) (Res, error)) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		res, err := fn(r.Context())
+		if err != nil {
+			Error(w, err)
+			return
+		}
+		JSON(w, http.StatusOK, res)
+	}
+}
+
+// HandleEmpty adapts a typed function with a body but no response body (returns 204).
+// Decodes JSON body into Req, validates, calls fn. Returns 204 on success.
+func HandleEmpty[Req any](v valid.Validator, fn func(ctx context.Context, req Req) error) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		var req Req
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			Error(w, badRequest("invalid JSON: "+err.Error()))
+			return
+		}
+		if err := v.Struct(req); err != nil {
+			Error(w, err)
+			return
+		}
+		if err := fn(r.Context(), req); err != nil {
+			Error(w, err)
+			return
+		}
+		NoContent(w)
+	}
+}
+
+// badRequest wraps a message in an ErrInvalidInput error.
+func badRequest(msg string) error {
+	return xerrors.New(xerrors.ErrInvalidInput, msg)
+}