fix(dashboard-api): async hygiene in routers/extensions.py

[yasinBursali]

What

Fixes three async-hygiene defects in routers/extensions.py: blocking urllib calls on the event-loop thread, over-broad except Exception catches that swallow programmer errors, and a fire-and-forget executor future that silently discards cleanup failures.

Why

• extension_logs and extensions_catalog called urllib.urlopen directly on the async event loop. The Console modal polls every 2 s with a 30 s agent timeout, so a single slow host-agent response could block the entire dashboard-api event loop for up to 30 s.
• _call_agent, _call_agent_invalidate_compose_cache, and _call_agent_compose_rename caught except Exception, meaning AttributeError, TypeError, and other programmer bugs were silently swallowed and logged as "host agent unreachable" — masking real bugs.
• _cleanup_stale_progress was dispatched via run_in_executor(None, ...) with the returned Future immediately discarded, so any unhandled exception inside the cleanup only produced an opaque "Future exception was never retrieved" warning in stderr with no context.

How

• Extracted _fetch_agent_logs(url, headers, data, timeout) -> str — a plain synchronous function that can be safely offloaded via asyncio.to_thread.
• extension_logs: replaced inline urllib.urlopen block with await asyncio.to_thread(_fetch_agent_logs, ...), matching the existing asyncio.to_thread pattern already used in main.py's api_settings_env_save.
• extensions_catalog: replaced direct _check_agent_health() call with await asyncio.to_thread(_check_agent_health).
• _call_agent, _call_agent_invalidate_compose_cache, _call_agent_compose_rename, _check_agent_health: narrowed except Exception → except (urllib.error.URLError, urllib.error.HTTPError, OSError, TimeoutError). Non-network exceptions now propagate with a full stack trace instead of a misleading warning.
• extensions_catalog: retained the run_in_executor Future in _cleanup_future and attached _log_cleanup_error as a done_callback to log any exception at ERROR level with full context.

Testing

• Automated: pytest 140/140 pass (137 pre-existing + 3 new); ruff clean.
• New tests (tests/test_extensions.py):
  • test_call_agent_returns_false_on_urlerror — network errors return False and log a warning.
  • test_call_agent_reraises_non_network_errors — AttributeError propagates (behaviour change from old broad catch).
  • test_catalog_logs_when_cleanup_future_fails — cleanup RuntimeError is logged at ERROR level, catalog endpoint still returns 200.
• Manual: Start dashboard-api with a non-responsive host agent; verify /api/extensions/catalog and /api/extensions/{id}/logs return promptly without blocking other requests.

Platform Impact

• macOS: Affected — dashboard-api runs in Docker on all platforms; async event-loop fix is platform-neutral.
• Linux: Affected — same fix applies to NVIDIA/AMD deployments.
• Windows (WSL2): Affected — same.

Known Considerations

Non-blocking follow-ups:

1. Add symmetric tests for _call_agent_invalidate_compose_cache and _call_agent_compose_rename (the other two narrowed helpers).
2. Hoist _log_cleanup_error out of the extensions_catalog function body to module level for reuse.
3. Audit remaining except Exception: blocks in main.py.