refactor: extract common logic in a separate class

Extract common semaphore logic into ConcurrentOperationBase for reuse in concurrent handlers.

WIP: correct version references and drop removal version

WIP: keep CSV download handler unthrottled, consistent with getCsvStreamResource

Author: mlopezFC

MIGRATION_GUIDE.md

@@ -2,7 +2,7 @@
 
 ## Overview
 
-Starting with version 2.6.0, Grid Exporter Add-on introduces new `DownloadHandler`-based methods to replace the deprecated `StreamResource` methods. This migration is necessary for forward compatibility: as of Vaadin 25, `StreamResource` is deprecated and marked for removal, and it will be removed in a future Vaadin release.
+Starting with version 3.1.0, Grid Exporter Add-on introduces new `DownloadHandler`-based methods to replace the deprecated `StreamResource` methods. This migration is necessary for forward compatibility: as of Vaadin 25, `StreamResource` is deprecated and marked for removal, and it will be removed in a future Vaadin release.
 
 ## What's Changing?
 
@@ -138,8 +138,8 @@ All existing features continue to work with the new API:
 
 ## Timeline
 
-- **Version 2.6.0**: New `DownloadHandler` methods introduced, old methods deprecated
-- **Version 3.0.0**: Old `StreamResource` methods will be removed
+- **Version 3.1.0**: New `DownloadHandler` methods introduced, old methods deprecated
+- **A future release**: Old `StreamResource` methods will be removed
 
 ## Need Help?
 
@@ -150,7 +150,7 @@ If you encounter any issues during migration, please:
 
 ## Example: Complete Migration
 
-**Before (Version 2.5.x):**
+**Before (Version 3.0.x):**
 ```java
 Grid<Person> grid = new Grid<>(Person.class);
 grid.setItems(people);
@@ -159,7 +159,7 @@ GridExporter<Person> exporter = GridExporter.createFor(grid);
 // Auto-attached buttons use StreamResource internally
 ```
 
-**After (Version 2.6.0+):**
+**After (Version 3.1.0+):**
 ```java
 Grid<Person> grid = new Grid<>(Person.class);
 grid.setItems(people);

README.md

@@ -61,9 +61,9 @@ To see the demo, navigate to http://localhost:8080/
 
 See [here](https://github.com/FlowingCode/GridExporterAddon/releases)
 
-## Migration to DownloadHandler API (Version 2.6.0+)
+## Migration to DownloadHandler API (Version 3.1.0+)
 
-**Important for Vaadin 25 compatibility**: Starting with version 2.6.0, Grid Exporter Add-on introduces new `DownloadHandler`-based methods to replace the deprecated `StreamResource` methods.
+**Important for Vaadin 25 compatibility**: Starting with version 3.1.0, Grid Exporter Add-on introduces new `DownloadHandler`-based methods to replace the deprecated `StreamResource` methods.
 
 ### Quick Migration
 
@@ -86,7 +86,7 @@ exporter.getCsvDownloadHandler()
 ### Requirements
 
 - **Vaadin 24.8.0 or later** is required to use the new `DownloadHandler` methods
-- Old `StreamResource` methods will continue to work until version 3.0.0
+- Old `StreamResource` methods will continue to work until they are removed in a future release
 - See [MIGRATION_GUIDE.md](./MIGRATION_GUIDE.md) for detailed migration instructions
 
 ## Issue tracking