Fix equality and hashcode contracts

[flyingsilverfin]

Usage and product changes

Streamline hash and equality contract differences:

• Java uses Java-defined implementations of both equality and hash code, instead of just delegating equality to Rust and doing hashcode on the Java side.
• Python's Datetime hash and equality both ignore timezone. Two Datetime objects representing the same instant in different timezones - following the Rust convention (as does CSharp. Java's native timezoned datetime that we defer to includes timezone in its contract)

Implementation

Java:

• Removed final from ConceptImpl.equals() so ValueImpl can override it
• Added ValueImpl.equals() using tryGetValueType().equals() && get().equals(), matching hashCode() which already uses get().hashCode()
• DatetimeTZ follows Java's ZonedDateTime.equals() semantics (local time + zone + offset)

Python:

• Removed _tz_name from Datetime.hash() tuple, restoring the hash-equality contract for timezone-aware datetimes