Reraise unexpected lexer argument errors

[ydah]

GraphQL::Language::Lexer#advance currently rescues every ArgumentError, but only handles errors whose message exactly matches "invalid byte sequence in UTF-8". Any other ArgumentError is silently swallowed and advance returns nil.

The exact message is produced while scanning an invalidly encoded string, so matching it also depends on Ruby's exception wording.

This PR validates the query string's encoding when the lexer is initialized and removes the broad rescue from advance. Invalid UTF-8 continues to raise the existing GraphQL::ParseError, while unrelated ArgumentErrors are propagated normally.

[rmosolgo]

Thanks for this improvement!