ci: bump actions/checkout from 6.0.2 to 6.0.3 #25

Job	Run time
Install fallback \| macos-latest \| py3.11	3s
Install pep517 \| windows-latest \| py3.11	3s
Install pep517 \| macos-latest \| py3.11	4s
Install fallback \| macos-latest \| py3.12	3s
Install pep517 \| ubuntu-latest \| py3.11	3s
Install pep517 \| ubuntu-latest \| py3.12	4s
Install fallback \| ubuntu-latest \| py3.12	3s
Install fallback \| ubuntu-latest \| py3.11	4s
Install fallback \| windows-latest \| py3.11	4s
Install pep517 \| windows-latest \| py3.12	4s
Install fallback \| windows-latest \| py3.12	4s
Install pep517 \| macos-latest \| py3.12	4s
Live Claude + Haiku validation (manual)	0s
API smoke \| ${{ matrix.os }} \| py${{ matrix.python-version }}	0s
	43s

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