docs(gateway-config): remove invalid sandbox_uid from VM driver example

[Bobbins228]

Summary

• Remove the commented-out sandbox_uid field from the VM driver TOML example in docs/reference/gateway-config.mdx
• VmComputeConfig uses deny_unknown_fields — uncommenting the line crashes the gateway with "unknown field sandbox_uid"
• The field exists on the internal VmDriverConfig subprocess (crates/openshell-driver-vm/src/driver.rs:231), set via --sandbox-uid CLI arg, not through the gateway TOML

Related Issue

Fixes #2364

Changes

Removed 3 lines (comment block + commented-out field) from the VM driver example in docs/reference/gateway-config.mdx. The Kubernetes driver example correctly retains sandbox_uid since KubernetesComputeConfig accepts it.

Testing

• Verified uncommenting the field causes gateway startup crash
• Verified the field is accepted on the openshell-driver-vm subprocess via --sandbox-uid
• mise run pre-commit passes (markdown lint, license check, helm lint)

Checklist

• Conventional commit format
• DCO sign-off
• No unrelated changes

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[Bobbins228]

I have read the DCO document and I hereby sign the DCO.

[Bobbins228]

recheck