Add more tests for parseutils.py

[rolandwalker]

Description

Add more tests for parseutils.py, covering SQL parsing, though there is some non-SQL as well. Added tests for

• extract_from_part()
• extract_table_identifiers()
• find_prev_keyword()
• get_last_select()
• is_subselect()
• is_valid_connection_scheme()
• last_word()
• query_is_single_table_update()

Also:

• adding the tests inspired catching a possible IndexError in query_is_single_table_update().
• removed an unused __main__ section from parseutils.py
• accidentally changed some quoting in the tests
• test_extract_columns_from_select() seemed to incorrectly catch an exception, which was removed. If tests can raise, that is something that should itself be tested, not covered up.

I couldn't figure out how to test the punctuation parameter to extract_from_part() and left a todo comment.

This is covered by an existing changelog entry.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Findings

1. Low: Missing regression test for the actual IndexError path this patch fixes.
   The new guard in query_is_single_table_update (try/except IndexError at line 382) is good, but the new parametrized test in test_parseutils.py does not cover malformed UPDATE ... WHERE ... statements that previously could trigger that path via is_destructive().
   Action: add a case like update test where id = 1 (and optionally is_destructive(["update"], "...UPDATE test WHERE id = 1;")) to lock in the fix.

No correctness/security regressions stood out in the diff beyond that testing gap.

Could not run tests in this environment because uv/pytest dependencies are unavailable.