Fix executemany in dbapi cursor implementation, make tests more comprehensive

[yunyu]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

executemany in the DBAPI cursor implementation would previously not work due to TypeError: can only concatenate str (not "bytes") to str in _do_execute_many. This PR correctly interprets everything as encoded bytes, and adds a test for executemany and parameterized queries.

(This unblocks something I ran into while getting chdb to work with clickhouse-sqlalchemy)

Documentation entry for user-facing changes

• [x] Documentation is written (mandatory for new features)

[yunyu]

@auxten @laodouya would you mind taking a look?

[auxten]

LGTM