Avoid double-free on duplicate enum names during BFBS deserialization

[steadytao]

Summary

Reject duplicate enum names during BFBS deserialisation before transferring ownership into the symbol table.

Problem

SymbolTable::Add() appended objects to its ownership vector before checking for duplicate names. During binary schema deserialisation, duplicate enum names could therefore leave a freed pointer behind in the symbol table and later trigger a double-free during teardown.

Fix

• check for duplicate names before appending the object to the symbol table's ownership vector
• add a regression test using a malformed BFBS fixture with duplicate enum names
• verify that VerifySchemaBuffer() accepts the schema but Parser::Deserialize() rejects it safely

Testing

• built & ran flattests.exe
• result: ALL TESTS PASSED

Fixes #9009

[steadytao]

CIFuzz found a real follow-up leak in Parser::StartEnum(): duplicate enum/union declarations allocate a temporary EnumDef and then return on duplicate-name rejection without freeing it. I’ve pushed a small follow-up onto this PR to delete that temporary object on the error path.