validate LocalArtifact data in API artifact creation (#173)

Ayush Sharmarequested to merge
ayushHardeniya/debusine:feat-173-api-artifact-validation into devel

The API endpoint did not run LocalArtifact Pydantic validation, while the web form already did.

this change invokes LocalArtifact._create() during API artifact creation, when the category corresponds to a LocalArtifact subclass, so inavlid data fails early with HTTP 400

adds tests for valid and invalid debian:lintian artifacts.

closes #173 (closed)

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