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
+}