Add datasource copy API

[guysmoilov]

Summary

• expose asynchronous datasource query materialization through copy_datasource and Datasource.copy_to
• serialize the current fluent datasource query automatically, including filters, projections/aliases, limits, and asOf
• return detailed datasource origin lineage from the copy mutation

Example:

materialized = source[source["score"] >= 0.8].select("label", "score").limit(10_000).copy_to(
    "high-confidence-training"
)

Tests

• GraphQL mutation and parameter serialization
• omitted-query latest-state behavior
• typed datasource/origin response conversion
• fluent Datasource.copy_to() query forwarding

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch data-engine-datasource-copy

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