UiEventHandler message limit is 1GB, accommodating JDK9+ UTF-8 parser

In JDK9+, the String(byte[], int, int, Charset) constructor may
allocate a new byte[] buffer with size twice the "length" parameter
if the Charset is UTF-8 and the input byte[] buffer contains a byte
with the high bit set.

If double the "length" integer parameter overflows, this leads to a
NegativeArraySizeException that crashes the build tool.

UI messages are already limited to just under 2GB to avoid common JVM
limitations. Since we do not expect UI messages between [1GB, 2GB) to
be useful for human consumption, and any such UI messages come from
stdout/stderr files that can be preserved for inspection, we can
reduce the existing limit from ~2GB to ~1GB.

PiperOrigin-RevId: 305930463