Fix premature COMMIT in stream-rows-to-copy under on error stop

[dimitri]

Fixes #1622 (specifically the hang scenario reported in the 2026-06-22 comment by fluca1978).

Root cause

In stream-rows-to-copy (used when on error stop is set), the original code placed COMMIT inside the unwind-protect cleanup alongside close-db-writer:

(unwind-protect
     (loop ...)
  (cl-postgres:close-db-writer copier)
  (pomo:execute "COMMIT"))  ; ← runs before the handler-case handler

In Common Lisp, handler-case handlers run after the dynamic extent of the protected form — which means after unwind-protect cleanup forms have completed. So when a data error occurred:

1. COMMIT ran inside the cleanup (wrong — partial data committed)
2. ROLLBACK ran in the handler-case handler (no-op / warning: no transaction in progress)

This left rows partially committed on error and put the connection into an inconsistent state. Under on error stop with concurrency > 1, the inconsistent connection state could cause the remaining writer threads to hang indefinitely (PG sessions stuck in ClientRead on COPY … FROM STDIN).

Fix

Move (pomo:execute "COMMIT") to after the unwind-protect so it only executes on the success path (loop and close-db-writer both complete without signalling):

(unwind-protect
     (loop ...)
  ;; Only close the COPY writer; cl-postgres with-syncing reads
  ;; ReadyForQuery on error, leaving the connection in SQL mode.
  (cl-postgres:close-db-writer copier))
;; Only reached on normal loop completion (no error).
(pomo:execute "COMMIT")

When either the loop or close-db-writer signals, the handler-case handler now finds a clean SQL-mode connection (cl-postgres's with-syncing in close-db-writer reads until ReadyForQuery before re-signalling) on which ROLLBACK works correctly.

A secondary fix guards (incf bytes row-bytes) with (when row-bytes …) so that a nil returned by stream-row for a filtered row does not raise a TYPE-ERROR that was itself masking the COMMIT sequencing issue.

Test

Added test/sqlite-on-error-stop.load which migrates test/sqlite/type-mismatch.db — a SQLite database whose products table has a TEXT value ('lots-of-it') stored in an INTEGER column. pgloader must exit cleanly without hanging. The test database is regenerated by test/sqlite/create-type-mismatch.py.