Defense-in-depth on top of _safe_session_filename_component (#5958):
Sink (makes the bad write impossible regardless of entry point):
- run_agent._save_session_log: sanitize session_id before building the
session_{sid}.json snapshot path.
- agent_runtime_helpers.dump_api_request_debug: sanitize before building
the request_dump_{sid}_{ts}.json path.
Boundary (clean 400 instead of a silently-hashed filename):
- api_server rejects path-traversal-shaped X-Hermes-Session-Id on the
session-continuation path and the explicit /api/sessions create path,
reusing gateway.session._is_path_unsafe (mirrors the native gateway's
entry-boundary guard). Also enforces the session-header length cap on
the continuation path.
Tests: traversal session_id stays contained at the write site; sanitizer
always yields a traversal-free segment; the API header rejects
../, absolute, and Windows-traversal IDs with 400.
ea1372d2af