3db3d60368
The session key construction logic was duplicated in 4 places (session.py + 3 inline copies in run.py), which is exactly the kind of drift that caused issue #349 in the first place. Extracted build_session_key() as a public function in session.py. SessionStore._generate_session_key() now delegates to it, and all inline key construction in run.py has been replaced with calls to the shared function. Tests updated to test the function directly.