feat(gpu): introduce GPU request spec

[elezar]

Summary

Replace legacy GPU-specific request fields in the public and driver protos with ResourceRequirements carrying a GPUSpec. This also adds GPU count requests while preserving the existing default GPU request shape used by --gpu and image-name auto-detection.

Related Issue

Related to #1444

Changes

• Added ResourceRequirements with an embedded GPUSpec to public and driver protos.
• Added optional GPUSpec.count, mutually exclusive with explicit device_ids.
• Mapped CLI GPU flags into ResourceRequirements: --gpu-device and --gpu-count now imply a GPU request, --gpu-count must be greater than zero, and --gpu-count conflicts with --gpu-device.
• Validated GPU count/device-ID mutual exclusion at the gateway and preserved GPU requirements in driver request translation.
• Honored GPU count in Kubernetes by setting nvidia.com/gpu to the requested count.
• Kept VM support limited to one GPU, and rejected count requests for Docker and Podman until those drivers support count semantics.
• Preserved image-name GPU auto-detection and --gpu as a present GPU request with no device IDs and no count.
• Updated unit tests, e2e fixtures, generated Python proto exports, and user-facing sandbox docs.
• Included a small VM clippy cleanup for the local container-engine fallback required by the current main baseline.

Testing

• mise run pre-commit passes
• cargo check -p openshell-core -p openshell-server -p openshell-driver-docker -p openshell-driver-kubernetes -p openshell-driver-podman -p openshell-driver-vm -p openshell-cli
• cargo test -p openshell-cli gpu
• cargo test -p openshell-core -p openshell-server -p openshell-driver-docker -p openshell-driver-kubernetes -p openshell-driver-podman -p openshell-driver-vm --lib gpu
• Unit tests added/updated
• E2E fixtures updated; dedicated GPU e2e coverage was already merged separately

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; user-facing docs updated)

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[github-actions]

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[github-actions]

Label test:e2e-gpu applied for cfceac9. Open the existing run and click Re-run all jobs to execute with the label set. The E2E Gate check on this PR will flip green automatically once the run finishes.