Fix Java ulong enum literal generation

[steadytao]

Summary

Emit Java ulong enum constants as signed long bit-equivalents instead of raw unsigned decimal literals.

Problem

For Java output, ulong enum values above Long.MAX_VALUE were emitted as decimal long literals such as:

public static final long None = 14695981039346656837L;

That does not compile in Java and fails with integer number too large.

Fix

• convert Java ulong enum constants to their signed long bit-equivalent before emitting the literal
• keep the generated type as long, which matches the existing Java representation for ulong
• add a focused Java regression schema and test for ulong enum constants above Long.MAX_VALUE

Testing

• reproduced the issue locally on current code generation
• confirmed the pre-patch generated Java fails to compile with integer number too large
• regenerated Java with the patch and confirmed it compiles cleanly
• added a regression test that checks the generated constants against Long.parseUnsignedLong(..., 16)

Fixes #8610.