Add update-check endpoint support (v0.2.1)

Implement /update-check endpoint aligned with remote OpenAPI spec:
- Add checkForUpdates() method to LicenseClientInterface
- Add UpdateInfo DTO for update check responses
- Add ProductNotFoundException for product_not_found error
- Update local openapi.json to v0.4.0

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.2.1] - 2026-01-27
+
+### Added
+
+- `checkForUpdates()` method for checking plugin updates
+- `UpdateInfo` DTO for update check responses
+- `ProductNotFoundException` for `product_not_found` error handling
+- `/update-check` endpoint support aligned with remote OpenAPI spec (v0.4.0)
+
+### Changed
+
+- Updated local `openapi.json` to match remote specification (now v0.4.0)
+- Added "Plugin Updates" tag to OpenAPI specification
+
 ## [0.2.0] - 2026-01-26
 
 ### Added