feat(Project): add MBR and hybrid volume schemas

[lengau]

This adds the ability to accept projects with mbr and mbr,gpt (hybrid) volume schemas. We don't implement this currently, we just accept projects that have it.

IMAGECRAFT-111

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[Copilot]

Pull request overview

This PR extends Imagecraft’s project model to accept additional volume schemas (mbr and mbr,gpt hybrid) by refactoring the volume schema into a discriminated union of concrete volume/structure types, and adds tests/fixtures to validate the new parsing behavior.

Changes:

• Refactor Volume into a discriminated union (GPTVolume, MBRVolume, HybridVolume) with schema-specific StructureItem types and validation.
• Update GPT-only packing/image-creation codepaths to type against GPTVolume while continuing to reject non-GPT schemas at image creation time.
• Add unit + integration test coverage and new valid/invalid project fixtures for MBR and hybrid schemas; adjust YAML reference generation inputs.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/unit/services/test_image.py	Update mocks to use GPT-specific structure item type.
tests/unit/pack/test_image.py	Switch volume parsing in tests to GPTVolume.
tests/unit/pack/test_grubutil.py	Switch volume parsing in tests to GPTVolume.
tests/unit/pack/test_gptutil.py	Switch volume parsing in tests to GPTVolume.
tests/unit/models/test_volume.py	Add extensive validation tests for MBR/hybrid and discriminated union behavior.
tests/unit/models/test_project.py	Add tests for partition-name generation across schema-specific structure items.
tests/integration/services/valid-projects/simple/imagecraft.yaml	Add a GPT-valid sample project for integration testing.
tests/integration/services/valid-projects/simple-mbr/imagecraft.yaml	Add an MBR-valid sample project for integration testing.
tests/integration/services/valid-projects/simple-hybrid/imagecraft.yaml	Add a hybrid-valid sample project for integration testing.
tests/integration/services/test_project.py	Add integration tests that load all valid/invalid project fixtures.
tests/integration/services/invalid-projects/mbr-unsupported-fields/imagecraft.yaml	Add invalid MBR fixture to ensure unsupported structure keys are rejected.
tests/integration/services/invalid-projects/mbr-unsupported-fields/error.txt	Expected error output for invalid MBR fixture.
tests/integration/services/invalid-projects/hybrid-unsupported-fields/imagecraft.yaml	Add invalid hybrid fixture to ensure unsupported structure keys are rejected.
tests/integration/services/invalid-projects/hybrid-unsupported-fields/error.txt	Expected error output for invalid hybrid fixture.
tests/integration/services/invalid-projects/gpt-unsupported-fields/imagecraft.yaml	Add invalid GPT fixture to ensure unsupported structure keys are rejected.
tests/integration/services/invalid-projects/gpt-unsupported-fields/error.txt	Expected error output for invalid GPT fixture.
tests/integration/conftest.py	Enable required craft-parts features for integration tests.
imagecraft/subprocesses.py	Capture stderr by default in the subprocess wrapper.
imagecraft/services/image.py	Cast GPT layouts to GPTVolume at the GPT-only image-creation callsite.
imagecraft/pack/gptutil.py	Type GPT utilities explicitly to GPTVolume; adjust partition JSON generation.
imagecraft/models/volume.py	Introduce schema-specific volume + structure item models and new schemas/roles.
imagecraft/models/init.py	Re-export GPT-specific models and update public model exports.
docs/reference/imagecraft-yaml.rst	Update kitbash-field references to GPT-specific model types.