implement OpenAPI contributor integration for rate limiting and response headers

flash-extensions/flash-ext-limiter/docs/annotation.md

@@ -92,6 +92,9 @@ public class PriceHandler extends RequestHandler { ... }
 Each annotation is processed by its own processor; Flash collects all middleware and
 composes them in processor registration order.
 
+When `flash-ext-openapi` is installed, `@Limit` also contributes OpenAPI response
+headers (`X-RateLimit-*`) and `Retry-After` on `429` automatically.
+
 ```java
 @Route(method = HttpMethod.DELETE, path = "/admin/users/{id}")
 @Limit(key = "auth_user", requests = 5, window = 1, windowUnit = TimeUnit.MINUTES)

flash-extensions/flash-ext-limiter/docs/http-headers.md

@@ -117,6 +117,12 @@ and **instead of** calling it on rejected requests. This means:
 
 ## Integration with Swagger UI (flash-ext-openapi)
 
-Rate-limit headers are not currently injected into the OpenAPI spec. If you want to
-document them, add them manually via `@ApiOperation` on the handler class using the
-response headers section of the OpenAPI spec.
+When `flash-ext-openapi` is installed, handlers annotated with `@Limit` automatically
+contribute rate-limit response headers to generated OpenAPI responses:
+
+- `X-RateLimit-Limit`
+- `X-RateLimit-Remaining`
+- `X-RateLimit-Reset`
+- `Retry-After` on `429`
+
+If `429` is not manually declared, OpenAPI auto-adds `429 Too Many Requests`.