fix(beam): normalize exception messages and string split separators

[dbrattli]

BEAM custom exceptions with numeric or union payloads expose Message as a raw Erlang value. String consumers and AggregateException formatting then fail with badarg. Route System.Exception.Message through a binary-producing runtime helper while preserving the original payload fields and existing string messages.

The string-consumer regressions also expose inconsistent String.Split separator normalization. A separate commit normalizes character and string separator arrays for plain, remove-empty, and count-limited splits.

Validation:

• ./build.sh test beam --force-fable-library: 2,736 .NET and 2,788 BEAM tests passed, including entry-point and compiler-diagnostic checks.
• Four exception regressions pass on .NET and reproduce badarg on the original BEAM runtime; three separator scenarios also reproduce badarg before the fix.
• Compiler formatting and git diff --check passed.