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_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)
+		}
+	})
+}