Allow cloud environments without Docker images

[warp-dev-github-integration]

Description

• Allow cloud environment payloads to omit the flattened docker_image field by representing the base image as optional.
• Preserve the existing docker_image wire format exactly when an environment pins an image.
• Make client display, copy, edit, and CLI paths handle unpinned environments without failing.
• Add regression coverage for missing images, pinned-image wire compatibility, and omission during serialization.

Linked Issue

N/A

Testing

• ./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 in the Warp client.

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

Conversation: https://staging.warp.dev/conversation/6495390f-cce5-413d-9ac4-0172f5ad2e4d
Run: https://oz.staging.warp.dev/runs/019f5d56-c242-73f3-9a58-51baaecfd33b

This PR was generated with Oz.