Catalog/metadata helpers reassigned cursor.fetchone/fetchmany/fetchall to closures on the instance, which made static type checkers infer a union of bound method and unbound function and report a spurious missing 'self' argument at call sites.

Route the catalog column map (including lowercase and specialized aliases) through the existing cached-map state that the standard fetch methods already consume, so metadata rows pick up catalog column names without any per-call method reassignment. Adds a regression test ensuring the fetch methods are never shadowed on the instance.

…ase=True (#620)

_prepare_metadata_result_set() hard-coded _cached_column_map_lower = None.
When lowercase=True the description column names are already lowercased, so
forcing the lowercase map to None dropped the Row case-insensitive fallback
and broke original-cased lookups (row.TABLE_NAME / row["TABLE_NAME"]) on
catalog result sets - a regression vs the previous wrapper-based code.

Build the lowercase map the same way execute() does (including any
specialized aliases). Also remove the now-dead self._column_map attribute
(nothing reads it; fetch methods use _cached_column_map) and add a
lowercase=True regression test for catalog column access.

cursor.py: _prepare_metadata_result_set() iterated self.description directly.
If DDBCSQLDescribeCol fails (swallowed above) and no fallback_description is
supplied (getTypeInfo() is the only such caller), description becomes None and
the loop raised TypeError. Iterate `self.description or ()` so the cursor yields
no rows instead. Not a pre-existing regression, but cheap to harden here.

test: the GH-620 shadowing test only compared row values (cursor.fetchall()
== [[1]]), which would not notice if the getTypeInfo() column-name cache leaked
into the following SELECT. Fetch the row and assert row.one == 1 and that
row.TYPE_NAME / row["TYPE_NAME"] raise, locking in the per-query cache rebuild.