load_session/save_session built the session file path directly from the
caller-supplied session_id with no validation, so a value like
'../../../tmp/secrets' (reachable from the 'load-session' CLI subcommand)
could read or write JSON files outside the session directory.
Add _validate_session_id() to reject anything outside [A-Za-z0-9_-]
(path separators, '..', absolute/Windows paths) before the id is used to
build a filename. Legitimate ids (uuid4 hex, 'session-<digits>-<n>') are
unaffected.
Adds SessionStorePathTraversalTests covering traversal rejection on both
load and save plus a legitimate-id round-trip.
Gracy769
mlliuz
陈家名
bellman