Fix missing instance lock in delete_fleets #6183

Job	Run time
compute-version	4s
build-docs / build-docs	1m 17s
build-artifacts / gateway-build	10s
build-artifacts / runner-test (macos-latest)	1m 12s
build-artifacts / frontend-build	12s
build-artifacts / code-lint	41s
build-artifacts / runner-test (ubuntu-latest)	55s
build-artifacts / python-build	19s
build-artifacts / generate-json-schema	18s
build-artifacts / python-test (macos-latest, 3.13)	3m 55s
build-artifacts / python-test (ubuntu-latest, 3.10)	5m 19s
build-artifacts / python-test (macos-latest, 3.11)	3m 14s
build-artifacts / python-test (macos-latest, 3.9)	4m 19s
build-artifacts / python-test (ubuntu-latest, 3.9)	5m 12s
build-artifacts / python-test (macos-latest, 3.12)	2m 44s
build-artifacts / python-test (windows-latest, 3.10)	1m 53s
build-artifacts / python-test (windows-latest, 3.12)	2m 4s
build-artifacts / python-test (ubuntu-latest, 3.13)	4m 39s
build-artifacts / python-test (windows-latest, 3.11)	2m 8s
build-artifacts / python-test (windows-latest, 3.9)	2m 9s
build-artifacts / python-test (ubuntu-latest, 3.12)	4m 56s
build-artifacts / python-test (macos-latest, 3.10)	2m 50s
build-artifacts / python-test (ubuntu-latest, 3.11)	4m 55s
build-artifacts / python-test (windows-latest, 3.13)	2m 5s
build-artifacts / runner-compile (ubuntu-24.04-arm, linux, arm64)	21s
build-artifacts / runner-compile (ubuntu-24.04, linux, amd64)	34s
upload-pre-pypi-artifacts / upload-pre-pypi-artifacts	18s
upload-post-pypi-artifacts / upload-post-pypi-artifacts	19s
	59m 2s

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