Allow cloud environments without a docker_image key

[warp-dev-github-integration]

Description

The client failed to deserialize cloud environments whose stored JSON has no docker_image key: AmbientAgentEnvironment used #[serde(flatten)] on the BaseImage enum, which made the top-level docker_image key required. Environments that don't pin a docker image were rejected and could not be loaded.

This PR makes the base image optional:

• AmbientAgentEnvironment.base_image is now Option<BaseImage>, serialized via a small base_image_field serde helper module (rename = "docker_image", default, skip_serializing_if) instead of #[serde(flatten)].
• Wire format is preserved exactly: environments with an image still serialize as a top-level "docker_image": "<image>" string; environments without one deserialize successfully (absent or null key) and re-serialize with the key omitted (no null, no empty string).
• All call sites that read base_image now handle the Option: CLI environment list/get output, the settings Environments page (card display and edit form), the conversation details panel (render and copy-to-clipboard), and the environment context-chip sidecar. Read-only surfaces show None when no image is pinned; the editable form field and clipboard copy use an empty string.

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).

Testing

Added unit tests in crates/cloud_object_models/src/cloud_environment_tests.rs:

• deserialize an environment without docker_image
• deserialize with an explicit null docker_image
• serialization omits the key when no image is set (and never emits a nested base_image object)
• serialization emits the exact legacy wire format when an image is set
• serde roundtrips both with and without an image

Validation run locally:

• cargo test -p cloud_object_models — 30 passed, 0 failed

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

• cargo clippy -p warp --lib -- -D warnings — clean

• cargo check -p warp --lib — clean

• ./script/format — no diffs

• 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 client.

Conversation: https://staging.warp.dev/conversation/ba380890-e12d-46e4-af8b-2be973f06b1d
Run: https://oz.staging.warp.dev/runs/019f5d57-d27e-70d7-b0be-adff71e67992

This PR was generated with Oz.