Retry image inspect without .Variant on template errors and add tests

[jooh]

Motivation

• Some engines return a template evaluation error when {{.Variant}} is absent, causing image inspection to fail during UID-update preparation.
• Ensure compose project naming covers the case where the compose file lives in the workspace root.

Description

• Add a fallback format constant UID_UPDATE_IMAGE_INSPECT_FORMAT_NO_VARIANT that omits {{.Variant}} and retry inspection when the template evaluation fails.
• Introduce is_missing_variant_template_error, inspect_image_details_without_variant, and factor parsing into parse_image_inspect_details to centralize inspect output handling.
• Update inspect_image_details_for_uid_update_once to detect the variant-template error and retry with the no-variant format.
• Add tests compose_project_name_defaults_to_compose_working_dir_basename and prepare_up_image_retries_image_inspect_without_variant_template_for_podman, and extend the fake engine fixture script to simulate variant vs. non-variant inspect outputs.

Testing

• Ran unit tests with cargo test for the devcontainer runtime package.
• The new tests compose_project_name_defaults_to_compose_working_dir_basename and prepare_up_image_retries_image_inspect_without_variant_template_for_podman executed and passed.
• Existing compose and UID-update related tests also executed and passed.

---

Codex Task

[jooh]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".