fix: normalize bridge errors and support nested group paths

Distinguish invalid KeePass requests from backend failures in the Python bridge, improve nested group path resolution, and add coverage for nested group creation plus payload forwarding.

.memory/todo.md

@@ -1,6 +1,5 @@
 # Todo
 
-- Improve failure-path coverage for write operations.
-- Verify and harden nested group path resolution.
-- Keep integration write tests on temporary copies only.
-- Keep API minimal and predictable.
+- Keep write-path behavior predictable and well-documented.
+- Preserve minimal API surface until update/delete/move is required.
+- Consider typed bridge errors in TypeScript if more granularity is needed later.