Implement feature installation order and enhance test suite

[prulloac]

This pull request introduces support for the installsAfter and dependsOn fields in the devcontainer feature specification, enabling both soft and hard dependencies between features. It also adds comprehensive internal tests to validate feature installation order logic, and updates documentation to reflect the new support status for these fields.

Feature dependency support:

• Added InstallsAfter (soft dependency) and DependsOn (hard dependency) fields to the Spec struct in features.go, allowing features to specify installation order preferences and requirements.

Testing improvements:

• Introduced devcontainer/install_order_internal_test.go with detailed tests covering canonical reference resolution, ambiguity handling, and correct enforcement of install order constraints, including interactions with pinned and free features.

Documentation updates:

• Updated docs/devcontainer-spec-support.md to mark overrideFeatureInstallOrder, dependsOn, and installsAfter as supported (with the appropriate status icon), reflecting the new implementation. [1] [2]

[Copilot]

Pull request overview

Adds support for devcontainer Feature installation ordering semantics, including a user-specified override order and dependency metadata read from devcontainer-feature.json, and extends the test suite and docs accordingly.

Changes:

• Add overrideFeatureInstallOrder to devcontainer.Spec and implement order resolution (resolveInstallOrder) plus dependency validation (validateDependencies).
• Extend devcontainer/features.Spec to parse installsAfter (soft ordering) and dependsOn (hard dependency).
• Add tests covering override ordering, installsAfter behavior, hard-dependency presence validation, and cycle detection; update spec support documentation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
docs/devcontainer-spec-support.md	Updates documented support matrix for devcontainer properties/features.
devcontainer/features/features.go	Adds JSON fields for installsAfter and dependsOn to feature metadata parsing.
devcontainer/devcontainer.go	Implements feature extraction pre-pass, install-order resolution, and hard dependency validation.
devcontainer/devcontainer_test.go	Adds tests for override ordering, installsAfter ordering, dependsOn validation, and cycle detection.

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