build(deps-dev): bump tsdown from 0.20.3 to 0.21.4

Bumps [tsdown](https://github.com/rolldown/tsdown) from 0.20.3 to 0.21.4.
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.20.3...v0.21.4)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.21.4
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package.json

@@ -79,7 +79,7 @@
     "axios": "1.13.6",
     "c8": "11.0.0",
     "jsdom": "28.1.0",
-    "tsdown": "0.20.3",
+    "tsdown": "0.21.4",
     "tslib": "2.8.1",
     "typescript": "5.9.3",
     "vite-bundle-analyzer": "^1.3.6",