Fix stall when TCP splits the trailing CRLF of an empty payload message (#1243)

* core: fix stall on split empty payload CRLF

ReadAtLeastAsync counts only the bytes read during the call, so asking
for 2 when the trailing \r is already buffered waits for a byte that
never comes. The read loop then stalls until unrelated traffic arrives,
which on an idle connection is the next keepalive PONG.

* Update tests/NATS.Client.Core2.Tests/SplitFrameTest.cs

Co-authored-by: synadia-claude-reviewer[bot] <265638981+synadia-claude-reviewer[bot]@users.noreply.github.com>

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Co-authored-by: synadia-claude-reviewer[bot] <265638981+synadia-claude-reviewer[bot]@users.noreply.github.com>