Separate statements in reindent_aligned output (issue673)

[adarshsm]

Fixes #673.

The bug

sqlformat -a (format(sql, reindent_aligned=True)) runs consecutive
statements together, so the second statement starts in the middle of the last
line of the first:

>>> sqlparse.format('select * from a;\nselect * from b;\n', reindent_aligned=True)
'select *\n  from a;select *\n  from b;'

Printed:

select *
  from a;select *
  from b;

reindent=True on the same input does the right thing:

>>> sqlparse.format('select * from a;\nselect * from b;\n', reindent=True)
'select *\nfrom a;\n\n\nselect *\nfrom b;'

Root cause

reindent_aligned turns on strip_whitespace (formatter.py), so by the time
AlignedIndentFilter runs, the whitespace that separated one statement from
the next has already been removed — and AlignedIndentFilter._process_statement
pops any remaining leading whitespace as well.

ReindentFilter.process() already compensates for exactly this, re-inserting a
newline before every statement after the first:

if self._last_stmt is not None:
    nl = '\n' if str(self._last_stmt).endswith('\n') else '\n\n'
    stmt.tokens.insert(0, sql.Token(T.Whitespace, nl))
self._last_stmt = stmt

AlignedIndentFilter.process() had no such step, so nothing put a separator
back.

The fix

Apply the same separator logic in AlignedIndentFilter.process(), reusing
ReindentFilter's rule so the two reindent modes agree. The filter is
constructed per format() call, so _last_stmt doesn't leak between calls —
same as in ReindentFilter.

After:

>>> sqlparse.format('select * from a;\nselect * from b;\n', reindent_aligned=True)
'select *\n  from a;\n\nselect *\n  from b;'

Single-statement output is unchanged.

Testing

• New TestFormatReindentAligned::test_multiple_statements_are_separated;
  verified it fails on master (from t1;select b on one line) and passes
  with the fix.
• Full suite: 507 passed, 2 xfailed, 1 xpassed. ruff check sqlparse/ clean.