Allow cloud environments without Docker images

[warp-dev-github-integration]

Description

Cloud environments stored without a docker_image currently fail to deserialize because the flattened BaseImage enum is required. This change models the flattened base image as optional so older or platform-specific environments without a pinned image load successfully.

Environments with a Docker image retain the exact existing wire shape ("docker_image": "..."). Environments without one deserialize to None, reserialize without the key, and omit image-only UI and CLI details.

This branch is based directly on 86dfca99cf45945233f72b25b100998856f29ec5.

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

No linked issue.

Testing

• Added unit coverage for deserializing a missing docker_image, round-tripping an absent image without emitting the field, and preserving the exact flattened JSON shape for a present image.

• ./script/format

• cargo test -p cloud_object_models

• cargo clippy -p cloud_object_models --all-targets --all-features --tests -- -D warnings

• cargo check -p warp --lib

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Fixed cloud environments without a pinned Docker image failing to load.

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/9a89c4f9-71c2-45e2-8b28-1c98c25c108b
Run: https://oz.staging.warp.dev/runs/019f5d58-f791-7679-9529-491a87f3d1b5

This PR was generated with Oz.