Four 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. The generated ExecStart pointed
to a Python binary without site-packages.
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 sometimes return empty after tool calls. The agent
silently abandoned the remaining work.
Fix: append assistant('(empty)') + user nudge message and retry
once. Resets after each successful tool round.
4. Compression model fallback on permanent errors (#8620 sub-issue 4)
When the default summary model (gemini-3-flash) returns 503
'model_not_found' on custom proxies, the compressor entered a
600s cooldown, leaving context growing unbounded.
Fix: detect permanent model-not-found errors (503, 404,
'model_not_found', 'no available channel') and fall back to
using the main model for compression instead of entering
cooldown. One-time fallback with immediate retry.
Test plan: 40 compressor tests + 97 gateway/CLI tests + 9 venv tests pass
772cfb6c4e