Add AWS Batch provider integration

[Hynek Kydlíček (hynky1999)]

Summary

• allow selecting AWS Batch for CPU cloud launches
• preserve the provider for retained debug sessions
• document AWS Batch launch and debug usage
• align the integrated sharding-limit assertion with the shared validator

Verification

• uv run ruff format --check and uv run ruff check on changed files
• uv run pytest tests/launchers/test_cloud_launcher.py (70 passed)
• uv run pytest (1,185 passed, 22 intentionally skipped)
• GitHub CI and CodeQL pass

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[chatgpt-codex-connector]

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Reviewed commit: 349ed5ce23

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