Add NewFormRequest for form-encoded HTTP requests

Creates requests with application/x-www-form-urlencoded body from
url.Values. Supports GetBody for retry compatibility, following the
same pattern as NewJSONRequest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

request.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 )
 
 // NewRequest creates an http.Request with context. It is a convenience
@@ -32,3 +33,20 @@ func NewJSONRequest(ctx context.Context, method, url string, body any) (*http.Re
 	}
 	return req, nil
 }
+
+// NewFormRequest creates an http.Request with a form-encoded body and
+// sets Content-Type to application/x-www-form-urlencoded.
+// The GetBody function is set so that the request can be retried.
+func NewFormRequest(ctx context.Context, method, rawURL string, values url.Values) (*http.Request, error) {
+	encoded := values.Encode()
+	b := []byte(encoded)
+	req, err := http.NewRequestWithContext(ctx, method, rawURL, bytes.NewReader(b))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.GetBody = func() (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(b)), nil
+	}
+	return req, nil
+}

request_form_test.go

@@ -0,0 +1,80 @@
+package httpx_test
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/url"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx"
+)
+
+func TestNewFormRequest(t *testing.T) {
+	t.Run("body is form-encoded", func(t *testing.T) {
+		values := url.Values{"username": {"alice"}, "scope": {"read"}}
+		req, err := httpx.NewFormRequest(context.Background(), http.MethodPost, "http://example.com/token", values)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		body, err := io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatalf("reading body: %v", err)
+		}
+
+		parsed, err := url.ParseQuery(string(body))
+		if err != nil {
+			t.Fatalf("parsing form: %v", err)
+		}
+		if parsed.Get("username") != "alice" {
+			t.Errorf("username = %q, want %q", parsed.Get("username"), "alice")
+		}
+		if parsed.Get("scope") != "read" {
+			t.Errorf("scope = %q, want %q", parsed.Get("scope"), "read")
+		}
+	})
+
+	t.Run("content type is set", func(t *testing.T) {
+		values := url.Values{"key": {"value"}}
+		req, err := httpx.NewFormRequest(context.Background(), http.MethodPost, "http://example.com", values)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		ct := req.Header.Get("Content-Type")
+		if ct != "application/x-www-form-urlencoded" {
+			t.Errorf("Content-Type = %q, want %q", ct, "application/x-www-form-urlencoded")
+		}
+	})
+
+	t.Run("GetBody works for retry", func(t *testing.T) {
+		values := url.Values{"key": {"value"}}
+		req, err := httpx.NewFormRequest(context.Background(), http.MethodPost, "http://example.com", values)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if req.GetBody == nil {
+			t.Fatal("GetBody is nil")
+		}
+
+		b1, err := io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatalf("reading body: %v", err)
+		}
+
+		body2, err := req.GetBody()
+		if err != nil {
+			t.Fatalf("GetBody(): %v", err)
+		}
+		b2, err := io.ReadAll(body2)
+		if err != nil {
+			t.Fatalf("reading body2: %v", err)
+		}
+
+		if string(b1) != string(b2) {
+			t.Errorf("GetBody returned different data: %q vs %q", b1, b2)
+		}
+	})
+}