[FEATURE] Coverage of full roundtrip conversion validation on all CocoIndex data types

[badmonster0]

A test util validate_full_roundtrip() was added in #594, which performs a full roundtrip of "Python encode -> Rust decode -> serialize -> deserialize -> Rust encode -> Python decode" and validates the value is unchanged. We need to apply it on all data types supported in CocoIndex, in test_convert.py

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[lemorage]

Working on this.