Fix segfault when closing cursor/connection after Statement.free()

[fdcastel]

Summary

• Fixes Segmentation fault when close connection or cursor. #65 — segfault (or DatabaseError: Invalid resultset interface) when closing a cursor or connection after the associated Statement has already been freed via context manager or explicit free() call.
• Root cause: Cursor._clear() unconditionally called self._result.close() on the IResultSet, even when the parent Statement had already been freed — operating on an invalidated Firebird interface.
• Fix: In Cursor._clear(), check whether the statement's interface (_istmt) is still valid before closing the result set. If the statement was already freed, safely discard the result set reference instead.
• Also fixes a minor bug in tests/conftest.py where --client-lib passed a Path instead of str to driver_config.fb_client_library.value.

Test plan

• test_issue_65_prepare_ctx_mgr — Statement freed via with context manager, then cursor closes
• test_issue_65_free_then_cursor_close — Explicit stmt.free() followed by cursor.close()
• test_issue_65_free_then_conn_close — stmt.free() followed by connection close (triggers cursor close via transaction rollback)
• All existing cursor and statement tests pass (23/23)