Refactor compose mount handling: unify mount definition and disable implicit workspace mounts

[jooh]

Motivation

• Simplify and unify how compose mount entries are represented and rendered by consolidating to a single mount definition type.
• Stop implicitly injecting a host workspace mount into compose override files for compose-based flows.
• Remove ad-hoc "short" mount syntax handling in favor of always using a normalized mapping shape.

Description

• Replaced the ComposeVolumeEntry enum and short-syntax handling with a single ComposeMountDefinition struct and updated APIs to return Vec<ComposeMountDefinition> from compose_additional_volumes and related helpers.
• Removed implicit workspace mount inclusion from compose_additional_volumes and removed compose_workspace_volume usage in override_file so workspace mounts are no longer added by default for compose flows.
• Updated YAML rendering functions in override_yaml to accept ComposeMountDefinition and render mapping entries directly, removing special-case short entry rendering.
• Adjusted mount parsing helpers in override_mounts to return the unified mount definition type and merged parsing/merging logic accordingly.
• Changed remote_workspace_folder_for_args in workspace.rs to prefer a configured workspaceFolder for compose flows and to ignore workspaceMount when determining the remote folder for compose-based flows.
• Updated unit tests and test expectations to reflect the new behavior and types, including renaming tests that assert workspace mounts are not added by default.

Testing

• Ran the updated unit test suites covering cmd/devcontainer/src/runtime/compose and cmd/devcontainer/src/runtime/context including mount parsing, YAML rendering, and compose override generation; all tests passed.
• Executed the modified tests that previously expected implicit workspace mounts to ensure they now assert the absence of those mounts and succeeded.

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Codex Task

[jooh]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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