* refactor(routes): extract the 585-line GET /api/session handler to _handle_session_get
Pure move out of handle_get's if/elif chain into a named module-level handler, matching the pattern the other ~200 routes already use. Body is verbatim (one-level dedent only); AST identity of the moved statements verified; no behavior change — the GET /api/session test slice passes identically before and after.
* test: repoint four source-window tests to _handle_session_get
The moved block's source-window extractors (chat_start synthesiser/read-only
pins, regressions runtime-streaming-state window, issue1436 load-path
markers, security-redaction inspect) now anchor on the extracted handler;
assertions unchanged.
* test: repoint messaging-session loader pin to _handle_session_get
---------
Co-authored-by: nesquena-hermes <nesquena+hermes@gmail.com>
