[tontester] Use unmangled name for JSON fields in TL objects

[DanShaders]

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[github-actions]

Issue found in commit 0c7c734:

• JSON key mismatch for mangled base-type fields: to_dict still uses arg.name for base types, while from_dict now reads arg.unmangled_name. For fields like from, bytes, self, list, serializing will emit from_ but deserializing expects from, breaking round‑trips and external JSON compatibility. Use arg.unmangled_name for keys in the base‑type branch too. File: test/tontester/src/tl/generator/generators/tlobject.py:152