Handle ignored inserts with explicit primary keys

[KairosOps]

Summary

This fixes the IndexError from #554 when a single-row insert(..., pk=..., ignore=True) is ignored after another table has advanced SQLite's connection-level lastrowid.

In that case cursor.lastrowid can point at a rowid from a different table, so looking up that rowid in the current table can return no rows. The insert already had an explicit primary key, so when SQLite reports rowcount == 0 for the ignored insert this now keeps last_pk based on the input record instead of trying to resolve it through the stale rowid.

Checks

• python -m pytest tests/test_create.py::test_insert_ignore tests/test_create.py::test_insert_ignore_with_pk_after_other_table_insert -q
• python -m pytest tests/test_create.py -q
• python -m py_compile sqlite_utils/db.py tests/test_create.py
• git diff --check
• python -m black --fast --check sqlite_utils/db.py tests/test_create.py

Refs #554

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📚 Documentation preview 📚: https://sqlite-utils--756.org.readthedocs.build/en/756/