Greptile review caught that /api/session handler calls _diag.maybe_start()
at entry and _diag.finish() only on the success path. Three exit paths
leaked the watchdog entry:
1. early-return when sid is missing (400)
2. early-return when session is not visible to active profile (409)
3. early-return when session not found (404)
4. the existing 'except KeyError:' branch at the bottom of the handler
Without _diag.finish() the watchdog's _watchdog_pending dict held the request
until the 5s slow-request timeout fired _on_timeout and emitted a spurious
'Slow WebUI request still running' log. The pending dict also stayed
unbounded by in-flight count.
Fix: every early-return path now calls _diag.finish() before returning;
the except KeyError branch calls it once at entry so all paths through
it are covered. finish() is idempotent (no-ops if already called).
Konstantin M
78dab75fc3