infra: add instructions for cleaning up testpypi artifacts (#2774)

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# Rationale for this change
Similar to apache/iceberg-rust#1855 

Add instructions to cleanup testpypi for `pyiceberg` in case of reaching
TestPyPI storage limits

## Are these changes tested?

## Are there any user-facing changes?

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Author: kevinjqliu

.github/workflows/nightly-pypi-build.yml

@@ -78,8 +78,35 @@ jobs:
     - name: List downloaded artifacts
       run: ls -R dist/
     - name: Publish to TestPyPI
+      id: publish-testpypi
+      continue-on-error: true
       uses: pypa/gh-action-pypi-publish@release/v1
       with:
         repository-url: https://test.pypi.org/legacy/
         skip-existing: true
         verbose: true
+    - name: Display error message on publish failure
+      if: steps.publish-testpypi.outcome == 'failure'
+      run: |
+        echo "::error::Failed to publish to TestPyPI"
+        echo ""
+        echo "⚠️  TestPyPI Publish Failed"
+        echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+        echo ""
+        echo "This may be due to TestPyPI storage limits."
+        echo "See: https://docs.pypi.org/project-management/storage-limits"
+        echo ""
+        echo "To resolve this issue, use the pypi-cleanup utility to clean up old TestPyPI artifacts:"
+        echo "https://pypi.org/project/pypi-cleanup/"
+        echo ""
+        echo "  uvx pypi-cleanup --package pyiceberg --host https://test.pypi.org/ \\"
+        echo "    --verbose -d 10 --do-it --username <username>"
+        echo ""
+        echo "Requirements:"
+        echo "  • Must be a maintainer for pyiceberg on TestPyPI"
+        echo "    (https://test.pypi.org/project/pyiceberg)"
+        echo "  • Requires TestPyPI password and 2FA"
+        echo "  • ⚠️  ONLY do this for TestPyPI, NOT for production PyPI!"
+        echo ""
+        echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
+        exit 1