Validate and route unknown connect() keywords

[vgvoleg]

Fixes #40.

connect() stored leftover keywords in connection_kwargs and never read them. Through a SQLAlchemy URL every query parameter lands there, so ?disable_discovery=true was silently dropped with no way to tell it apart from an option that did not help.

Changes

• prepare_driver_config_kwargs() in utils.py: routes leftover keywords into ydb.DriverConfig kwargs, resolving valid names from inspect.signature(ydb.DriverConfig.__init__) rather than a local list that would go stale as the SDK gains parameters.
• Coerces string values by parameter annotation. URL query params arrive as strings, so ?disable_discovery=false was a truthy string and disabled discovery.
• Raises ProgrammingError for unknown names, unparseable values, and options passed both directly and via driver_config_kwargs.
• Driver options combined with ydb_session_pool now raise: no DriverConfig is built there, so accepting them would keep the same silent behavior.
• auth_token together with credentials now raises. DriverConfig lets auth_token overwrite credentials; the keyword used to be dropped entirely, so this guard comes with making it live.

_additional_sdk_headers is untouched apart from being popped earlier, before validation runs — otherwise it would trip the new check on the shared-pool path.

Behavior change

Keywords that are silently ignored today start raising, including user:password@ in a URL (not a documented form in ydb-sqlalchemy). driver_config_kwargs passed alongside ydb_session_pool was ignored and now raises.

Verification

poetry run poe tests — 131 passed. poetry run poe lint green. Checked against a local YDB with the exact keywords SQLAlchemy produces: disable_discovery="true" → True, "false" → False, typos and username/password → ProgrammingError.