refactor(compute): remove legacy host_users platform configuration

[drew]

Description

Remove the temporary DriverSandboxTemplate.platform_config["host_users"] compatibility encoding at the coordinated 0.1.0 API stability boundary. The public compute-driver contract should carry portable user-namespace intent only through the typed optional DriverSandboxTemplate.user_namespaces field.

Context

PR #2823 moves Kubernetes-specific hostUsers translation into the Kubernetes compute driver and adds the typed user_namespaces field to compute_driver.proto. During the transition, the gateway continues emitting the inverse legacy platform_config.host_users value for older external Kubernetes driver binaries, and the Kubernetes driver continues accepting that value from older gateways.

This compatibility shim deliberately preserves mixed-version operation, but the opaque field is Kubernetes-specific and should not remain part of the stabilized compute-driver contract. This work is tracked under #2565 as part of the final coordinated breaking-API pass for 0.1.0.

Definition of Done

• Remove gateway emission of DriverSandboxTemplate.platform_config["host_users"].
• Remove the Kubernetes driver's fallback parsing of platform_config.host_users.
• Keep DriverSandboxTemplate.user_namespaces as the sole compute-driver representation of portable user-namespace intent.
• Update compatibility tests to enforce the post-transition typed-only contract.
• Document the removal in the 0.1.0 migration notes and compute-driver compatibility matrix.
• Confirm all first-party and supported external compute drivers consume the typed field before removal.

Dependency

Do not remove the shim until the project deliberately enters the 0.1.0 breaking-change window and the supported external-driver compatibility policy from #2565 permits it.