COPY TO STDOUT misreads data after a notice longer than the read buffer #6702

NpgsqlRawCopyStream, which BeginTextExport reads through, and NpgsqlBinaryExporter keep the read buffer they found when they were created (NpgsqlRawCopyStream.cs:73, NpgsqlBinaryExporter.cs:62). A notice longer than the read buffer (8 KB by default) makes the connector switch to a bigger one (NpgsqlConnector.cs:1523-1545), and the reader then reads from the old, emptied buffer, i.e. directly from the socket, past what the connector has already received.

CREATE FUNCTION pg_temp.noisy(i int, len int) RETURNS int LANGUAGE plpgsql AS $$
BEGIN RAISE NOTICE '%', repeat('x', len); RETURN i; END $$;
-- then: COPY (SELECT pg_temp.noisy(i, 20000) FROM generate_series(1, 3) i) TO STDOUT, with and without (FORMAT BINARY)

On PostgreSQL 18.6 with Command Timeout=5:

• BeginRawBinaryCopy returned 29 bytes of the notice text as COPY data, then failed with "Unknown message code: 80";
• BeginBinaryExport failed with "Invalid COPY binary signature at beginning!";
• BeginTextExport returned nothing, timed out after 5 s and left the connection broken.

With len = 10 all three read 1, 2 and 3. If an earlier message had already made the connector switch, the next switch returns that buffer's array to the ArrayPool while the reader goes on reading socket data into it, even after the pool has handed it out again (shown with a mock probe). The same code is on the 8.0, 9.0 and 10.0 hotfix branches.