Three independent fixes:
1. Reset activity timestamp on cached agent reuse (#9051)
When the gateway reuses a cached AIAgent for a new turn, the
_last_activity_ts from the previous turn (possibly hours ago)
carried over. The inactivity timeout handler immediately saw
the agent as idle for hours and killed it.
Fix: reset _last_activity_ts, _last_activity_desc, and
_api_call_count when retrieving an agent from the cache.
2. Detect uv-managed virtual environments (#8620 sub-issue 1)
The systemd unit generator fell back to sys.executable (uv's
standalone Python) when running under 'uv run', because
sys.prefix == sys.base_prefix (uv doesn't set up traditional
venv activation). The generated ExecStart pointed to a Python
binary without site-packages, crashing the service on startup.
Fix: check VIRTUAL_ENV env var before falling back to
sys.executable. uv sets VIRTUAL_ENV even when sys.prefix
doesn't reflect the venv.
3. Nudge model to continue after empty post-tool response (#9400)
Weaker models (GLM-5, mimo-v2-pro) sometimes return empty
responses after tool calls instead of continuing to the next
step. The agent silently abandoned the remaining work with
'(empty)' or used prior-turn fallback text.
Fix: when the model returns empty after tool calls AND there's
no prior-turn content to fall back on, inject a one-time user
nudge message telling the model to process the tool results and
continue. The flag resets after each successful tool round so it
can fire again on later rounds.
Test plan: 97 gateway + CLI tests pass, 9 venv detection tests pass
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