fix(api): reject >20MB single_part uploads with a clear error

UploadFile always sends mode=\"single_part\" but Notion's single-part
upload limit is ~20MB; anything larger fails on the server with an
opaque error. The client already knows the file size, so add an
explicit SinglePartUploadMaxBytes constant and fail the upload early
with a message that names the limit and notes that multi_part uploads
are not yet supported. Full multipart support is out of scope for
this PR and tracked as follow-up work.

Author: 0xble

internal/api/client.go

@@ -134,6 +134,11 @@ func (c *Client) GetPageMarkdown(ctx context.Context, pageID string) (*PageMarkd
 	return &out, nil
 }
 
+// SinglePartUploadMaxBytes is Notion's single_part file-upload size limit.
+// Files larger than this require the multi_part upload flow, which this
+// client does not yet implement.
+const SinglePartUploadMaxBytes = 20 * 1024 * 1024
+
 // UploadFile streams a file upload to the Notion API without buffering the whole
 // payload in memory. The reader is consumed through a multipart writer piped
 // directly into the HTTP request body. Pass the exact byte size so the server
@@ -146,6 +151,9 @@ func (c *Client) UploadFile(ctx context.Context, name string, size int64, body i
 	if size <= 0 {
 		return "", fmt.Errorf("file size must be positive")
 	}
+	if size > SinglePartUploadMaxBytes {
+		return "", fmt.Errorf("file %q is %d bytes; Notion's single_part upload limit is %d bytes (~20MB) and multi_part uploads are not yet supported by this client", name, size, SinglePartUploadMaxBytes)
+	}
 	if body == nil {
 		return "", fmt.Errorf("file body is required")
 	}

internal/api/client_test.go

@@ -260,3 +260,24 @@ func TestTrashPageUsesPatch(t *testing.T) {
 		t.Fatalf("TrashPage: %v", err)
 	}
 }
+
+func TestUploadFileRejectsOversizedSinglePart(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatalf("unexpected request: %s %s", r.Method, r.URL.Path)
+	}))
+	defer srv.Close()
+
+	client, err := NewClient(config.APIConfig{BaseURL: srv.URL + "/v1"}, "secret-token")
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+
+	oversize := int64(SinglePartUploadMaxBytes + 1)
+	_, err = client.UploadFile(context.Background(), "big.png", oversize, strings.NewReader(""))
+	if err == nil {
+		t.Fatalf("UploadFile returned nil error; expected size-limit error")
+	}
+	if !strings.Contains(err.Error(), "single_part upload limit") {
+		t.Fatalf("UploadFile error = %q, want single_part limit message", err.Error())
+	}
+}