feat: Remove Exceptions

Generated `parseImpl` no longer declares `throws ParseException/IOException`; error paths set a sticky error code and return null instead. Public throwing API unchanged.

**Benefit:** removes exception-based control flow from the generated hot path, which is expensive in Java.

PR: hashgraph/pbj#915

Start by reading the generated parseImpl entry point and PR hashgraph/pbj#915, then trace how its error paths set the sticky error code. Done means generated parseImpl no longer declares ParseException or IOException, returns null on those error paths, and the public throwing API remains unchanged.

Written by the indexing model from the issue text.