fix: add regression test guarding devcontainer-lock.json JSON validity

[Copilot]

The build-and-test devcontainer CI job was failing because a stray } was appended to .devcontainer/devcontainer-lock.json, making it invalid JSON. The devcontainers CLI aborted immediately with SyntaxError: Unexpected non-whitespace character after JSON at position 2143 (line 35 column 3) — before even attempting to build or run tests.

Changes

• Structural fix (PR fix: remove stray closing brace corrupting devcontainer-lock.json #852, already merged): Removed the stray closing brace from devcontainer-lock.json.
• Regression test (tests/unit/test_devcontainer_json.py): Three unit tests that validate both .devcontainer/devcontainer.json and devcontainer-lock.json parse as valid JSON and that the lock file has the expected top-level features key. These run in the fast unit suite (python scripts/test_runner.py --unit) and will catch this class of corruption before it reaches CI.

[Copilot]

@Bryan-Roe Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository and had to close this PR. The error I am seeing is:

no `copilot-setup-steps` job found in your `copilot-setup-steps.yml` workflow file. Please ensure you have a single job named `copilot-setup-steps`. For more details, see https://gh.io/copilot/actions-setup-steps

Once you or someone with the necessary access fixes the problem, please unassign and then reassign issue #0 to me and I'll retry. Thanks!

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[Copilot]

@Bryan-Roe Thanks for sending me some feedback. Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository. The error I am seeing is:

no `copilot-setup-steps` job found in your `copilot-setup-steps.yml` workflow file. Please ensure you have a single job named `copilot-setup-steps`. For more details, see https://gh.io/copilot/actions-setup-steps

Once you or someone with the necessary access fixes the problem, please let me know in a comment and I'll try again. Thanks!

[Copilot]

Pull request overview

Adds regression coverage preventing malformed devcontainer JSON from reaching CI.

Changes:

• Validates both devcontainer JSON files parse successfully.
• Verifies the lock file contains its required features key.