Support optional argument prefixes for container runtimes

[Shawn Yuan (shuaiyuanxx)]

Summary

Adds an optional, repeatable --runtime-arg option to set-up and run-user-commands.

This enables Dev Containers to target existing WSLC SDK Sessions using wslc --session <name>.

This PR needs to be used together with this dev container PR

Changes

• Preserve runtime arguments through setup, lifecycle commands, runtime probes, and internal exec/PTY operations.
• Keep existing behavior when the option is omitted.
• Limit support to the two commands used by the attachment flow.

Example

devcontainer set-up \
  --docker-path wslc \
  --runtime-arg=--session \
  --runtime-arg="SDK Session" \
  --container-id CONTAINER_ID

Validation

• Focused tests, type checking, lint, and build passed.
• Verified both commands against an SDK-created container.
• Verified missing-session rejection and VS Code attachment, terminal access, and reload with the paired extension.