Accept cloud environments without a pinned docker image

[warp-dev-github-integration]

Description

The client rejected cloud environments whose stored JSON has no docker_image field: AmbientAgentEnvironment.base_image was a required, flattened BaseImage enum, so deserialization failed when the key was absent and the environment silently failed to load.

This PR makes base_image an Option<BaseImage> (still #[serde(flatten)]), so:

• Environments without a pinned docker image now deserialize with base_image: None (serde's flattened-Option handling yields None when no variant key is present).
• The wire format for environments that do pin an image is unchanged: a flat top-level docker_image key, and None serializes to no key at all.

Consumers updated to handle the absent image:

• warp agent environment CLI (list/get/update) prints/serializes the image only when present ("None" in tables/details).
• Settings → Environments list/edit form omit the "Image:" detail when unset.
• Conversation details panel only renders/copies the Image field when present.
• Environment picker sidecar shows "(none)" when no image is pinned.

Linked Issue

N/A — client-side deserialization fix.

Testing

• Added unit tests in crates/cloud_object_models/src/cloud_environment_tests.rs:
  • deserializing an environment without docker_image succeeds with base_image: None
  • serializing base_image: None omits the docker_image key
  • serializing a pinned image preserves the exact flat docker_image wire format
  • roundtrip without a docker image
• cargo test -p cloud_object_models — all pass
• cargo clippy -p cloud_object_models --all-targets --all-features --tests -- -D warnings — clean
• cargo clippy -p warp --all-targets -- -D warnings — clean
• cargo check -p warp --lib — compiles
• ./script/format — 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 client.

Conversation: https://staging.warp.dev/conversation/b6751d1c-549b-4a27-a268-5b864c2e9a7e
Run: https://oz.staging.warp.dev/runs/019f5d56-7653-71ca-976c-683147508672

This PR was generated with Oz.