Strip all whitespace before commas in reindented identifier lists

[sarathfrancis90]

Summary

Reindenting an identifier list is not idempotent when a comma is preceded by
more than one whitespace token. Re-formatting the formatter's own output
changes it, which it should not:

>>> import sqlparse
>>> a = sqlparse.format("select a  ,  b from t", reindent=True)
>>> a
'select a ,\n       b\nfrom t'          # note the stray space before the comma
>>> sqlparse.format(a, reindent=True)
'select a,\n       b\nfrom t'          # the space is gone on a second pass

So format(format(sql)) != format(sql). The single-whitespace case
(select a , b) already hugs the comma (a,) and is idempotent, which shows
the stable output the multi-whitespace case should also produce.

Root cause

StripWhitespaceFilter._stripws_identifierlist (sqlparse/filters/others.py)
removes whitespace before commas (the original fix for issue140), but only
tracked the single most recent whitespace token:

last_nl = None
for token in list(tlist.tokens):
    if last_nl and token.ttype is T.Punctuation and token.value == ',':
        tlist.tokens.remove(last_nl)
    last_nl = token if token.is_whitespace else None

When a comma is preceded by several whitespace tokens (multiple spaces, tabs,
or a newline followed by a space), only the last one is removed. The remaining
whitespace token is then collapsed to a single space by
_stripws_default, leaving the stray a ,.

Fix

Track the full run of consecutive whitespace tokens before a comma and remove
all of them, so the comma always hugs the preceding token:

last_ws = []
for token in list(tlist.tokens):
    if last_ws and token.ttype is T.Punctuation and token.value == ',':
        for ws in last_ws:
            tlist.tokens.remove(ws)
    last_ws = last_ws + [token] if token.is_whitespace else []

Only whitespace before a comma is removed, so comma_first spacing
(whitespace after a comma) is unaffected.

As a beneficial side effect this also resolves the extra-space output reported
in #644 (comma_first / reindent_aligned producing select * ,).

Test evidence

Added TestFormatReindent.test_identifier_list_whitespace_before_comma
covering multiple spaces, several spaces, tabs, and newline+space before a
comma, asserting both the expected output and idempotency. The new test fails
on the current code and passes with the fix.

$ pytest -q
486 passed, 2 xfailed, 1 xpassed
$ ruff check sqlparse/
All checks passed!

Checklist

• ran the tests (pytest)
• all style issues addressed (ruff)
• your changes are covered by tests
• your changes are documented, if needed (CHANGELOG entry added)