Fix NpgsqlRawCopyStream's dispose and later reads after an export error - #6711

When the server reported an error after CopyDone instead of CommandComplete (from an AFTER trigger such as a foreign key check, from a deferred constraint at commit, or from a cancellation while these run), the raw COPY stream threw it, but it read CommandComplete and ReadyForQuery outside the cleanup that a failed read of a CopyData message gets. The stream stayed ready, although the connector had already read up to the ReadyForQuery, so its dispose, which closing the connection runs too, waited for another ReadyForQuery, and so did a later read: until the command timeout broke the connection (which also cleared the data source's pool, unless Cancellation Timeout is -1), or forever without one. An error from a second statement in the command, instead of ReadyForQuery, did the same. BeginTextExport reads through the raw stream, so its reader had the same problem.

These reads now end the export as a failed read of a CopyData message does: the stream releases the connection at once, so its dispose neither skips nor waits, and the COPY activity records the error instead of the timeout. After an error ended an export at a read (Cancel makes the server send one too), later calls until the dispose throw InvalidOperationException "The COPY operation ended with an error." with the error as the inner exception: on the raw stream Read, ReadAsync, ReadByte, Write, Flush and Cancel, on the text reader its reads and Cancel. Before, they threw ObjectDisposedException without the error, or waited as above. After the dispose they throw ObjectDisposedException, as before. The stream no longer logs that the COPY operation completed after such an error, nor when BeginRawBinaryCopy, BeginTextExport or BeginTextImport fails before the COPY starts, e.g. when the server answers the command with an error for a missing table.

How a real server gets there

PostgreSQL sends CopyDone before EndCopyTo runs ExecutorFinish, which fires the queued AFTER triggers (copyto.c:1118 and :1015, copy.c:358-359, execMain.c:443), and it commits before it reports command completion (postgres.c:1284-1298 and :1337; all REL_18_STABLE).

CREATE TEMP TABLE parent (id int PRIMARY KEY);
CREATE TEMP TABLE child (id int REFERENCES parent);
BEGIN;
COPY (INSERT INTO child VALUES (1) RETURNING id) TO STDOUT;  -- BeginTextExport; BeginRawBinaryCopy with (FORMAT BINARY)

Live on PostgreSQL 18.6, Debug and Release, sync and async, Command Timeout 5 s:

• Without this change: the read threw 23503 at once; the dispose then waited 5 s, the connection was broken, and the pool's idle connection was gone. The same with a deferred constraint at commit, a plain AFTER ROW trigger, statement_timeout during a statement-level AFTER trigger and a failing second statement, for BeginRawBinaryCopy and BeginTextExport alike.
• With this change: the read throws the original at once, later calls throw the InvalidOperationException until the dispose, the dispose returns at once, the connection serves ROLLBACK, RESET and SELECT 1, and the pool's idle connection is reused. Also for an error at a row and for Cancel during a long export. The COPY activity records the 23503 instead of the dispose's timeout.
• Begin failures: a missing table (42P01), a bad column or option, an aborted transaction (25P02), a COPY to or from a file on the server, and pg_terminate_backend during a lock wait no longer log "completed". The exception, the activity and the connection are unchanged.

Tests

• CopyTests.Raw_export_error, 40 mock cases: raw binary and text export; an error before the first row, before the second, after CopyDone, after CommandComplete, or none; with and without a later read; sync and async.
• TracingTests.RawBinaryExport_error_after_CopyDone (2 cases), the only test of the catch's TraceSetException.
• CopyTests.Raw_copy_error_at_start, 16 cases: all four APIs, sync and async, with an ErrorResponse 42P01 instead of the COPY response, or as a control that disposes right after a successful Begin.
• Red first, against the stream before this change: 34 of the first 42 cases fail, and the 8 no-error controls among them pass, in 3 of 3 runs each in Debug and Release. Of Raw_copy_error_at_start, exactly the 8 error cases fail without the new log condition. Seven mutations of the change were each caught by exactly the expected cases.

For the reviewer

• A core change in NpgsqlRawCopyStream, with no API change. The message text and the field _error match Fix NpgsqlBinaryExporter's dispose after an error or a connection break #6713.
• ObjectDisposedException derives from InvalidOperationException, so catch (InvalidOperationException) still catches both, while catch (ObjectDisposedException) no longer catches the new exception before the dispose.
• After Cancel, the server answers with 57014, and the read turns it into NpgsqlException with an inner TimeoutException, as before; later calls now carry that exception as the inner exception, and "completed" is no longer logged.
• The Begin failure check is "Init threw", not "consumed": imports never reach the consumed state, and this class logs an export disposed early as completed. "Completed" is still logged after an early dispose that swallows an error and after breaks (pre-existing).
• The raw and text exports release the connection at the failing read, as they already did for an error instead of CopyData, while the binary exporter keeps the Copy state until its dispose (Fix NpgsqlBinaryExporter's dispose after an error or a connection break #6713). Both patterns predate these PRs; neither waits.
• Backport: planned for the 10.0 hotfix branch, after Fix NpgsqlBinaryExporter's dispose after an error or a connection break #6713's. The source merges cleanly onto 10.0, and the mock tests need Add copy-both and replication support to the test server mock #6689's mock writers there.

Dependencies and merge order

• It depends on no other PR.
• No textual conflict with Read COPY data from the connector's current read buffer #6712 or Fix NpgsqlBinaryExporter's dispose after an error or a connection break #6713; any order.

Fixes #6704