Fix broken default dialect behavior

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: Fixes sourcemeta::codegen::compile so schema canonicalization respects the provided default dialect/ID when a schema omits $schema.

Changes:

• Passes `default_dialect`/`default_id` through to `SchemaTransformer::apply` during canonicalization
• Adds a regression test covering the `default_dialect` parameter for draft 2020-12

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[augmentcode]

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