Preserve single grouped expressions in function parameters

[emme1t]

Function.get_parameters() returns an empty list for single expression arguments such as abs(1 + 2), abs((1 + 2)), abs(CASE WHEN 1 THEN 2 ELSE 3 END), and abs(1 = 2).

These arguments have already been grouped into Operation, Parenthesis, Case, and Comparison nodes. Include those node types in the existing single-argument collection path. The regression tests verify that each complete argument is returned as one node with its original text and type.

Validation on Windows with Python 3.13.13:

• All four new regression cases fail on the original implementation.
• Full suite: 510 passed, 2 xfailed, 1 xpassed. A clean checkout of the original commit has the same expected-failure statuses: 506 passed, 2 xfailed, 1 xpassed. The existing XPASS is test_issue484_comments_and_newlines.
• ruff check sqlparse and git diff --check pass.
• All four example expressions were independently executed using SQLite 3.50.4's single-argument abs() function; they return 3, 3, 2, and 0 respectively. The parsed argument lists now contain the corresponding single expressions.

OpenAI Codex assisted with the source review, implementation, regression tests, and this description. The reported tests and SQLite checks were executed locally.