Fixes the multi-client profile isolation bug (#798).
- get_hermes_home_for_profile(): pure path resolver, validates name against
_PROFILE_ID_RE (rejects path traversal), never mutates os.environ or globals
- new_session() accepts explicit profile= param from POST body (S.activeProfile),
short-circuits the process-level _active_profile global
- streaming handler resolves HERMES_HOME from s.profile instead of the global
- sessions.js sends profile: S.activeProfile in every new-session POST
10 tests in tests/test_issue798.py including concurrency and traversal coverage.
Co-authored-by: nesquena <nesquena@users.noreply.github.com>
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