feat: mediate creation of gadget.yaml and kernel.yaml

[cmatsuoka]

Create the gadget.yaml and kernel.yaml metadata files using the new
packaging API to skip repacking when contents were not changed.

---

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[Copilot]

Pull request overview

This PR updates Snapcraft’s core24+ (“craft-application”) packaging path to mediate creation of meta/gadget.yaml, meta/kernel.yaml, and meta/component.yaml contents, aiming to avoid unnecessary repacks when those files’ contents have not changed.

Changes:

• Add mediated generators in the Package service for meta/component.yaml, meta/gadget.yaml, and meta/kernel.yaml, and materialize gadget/kernel YAML into the prime meta/ directory during write_metadata().
• Adjust setup_assets() to only copy gadget/kernel YAML via the legacy path (core22), and add/extend unit tests for these behaviors.
• Refactor component metadata generation to support rendering component.yaml as a string (get_str) and bump craft-application dependency.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
uv.lock	Bumps craft-application dependency constraint to support new packaging behavior.
tests/unit/services/test_package.py	Adds coverage for gadget/kernel mediation behavior and write_metadata() output.
tests/unit/services/test_package_components.py	Adds coverage for component mediation partition naming/registry behavior.
tests/unit/parts/test_setup_assets.py	Ensures core24 no longer copies gadget/kernel YAML in legacy asset setup path.
tests/unit/models/test_assertions.py	Removes ty: ignore around ValidationAssertion construction.
tests/unit/meta/test_component_yaml.py	Adds coverage for component_yaml.get_str().
snapcraft/services/package.py	Introduces mediated generators and writes gadget/kernel YAML into prime/meta/.
snapcraft/parts/setup_assets.py	Adds legacy gating helper and conditionalizes gadget/kernel YAML copying.
snapcraft/meta/component_yaml.py	Refactors component metadata generation into get_metadata() + get_str() and updates write().
snapcraft/commands/validations.py	Removes ty: ignore around ValidationAssertion construction.

[mr-cal]

LGTM, but I think you need to do a make format.

[mr-cal]

This should simplify it:

Suggested change

	return project.base == "core22" or (
	project.base is None and project.build_base == "core22"
	)
	return project.get_effective_base() == "core22"