test(tasks): add preflight fallback coverage to scheduler tests

Adds test_detect_vram_preflight_fallback to cover the spec path where
cf-orch is unreachable but scripts.preflight.get_gpus() succeeds,
verifying detect_available_vram_gb() returns the summed total VRAM.
Uses sys.modules injection to simulate the preflight module being present.

tests/test_tasks/test_scheduler.py

@@ -5,6 +5,7 @@ import sqlite3
 import threading
 import time
 from pathlib import Path
+from types import ModuleType
 from typing import List
 from unittest.mock import MagicMock, patch
 
@@ -88,6 +89,28 @@ def test_detect_vram_cforch_empty_nodes_falls_back():
     assert result == 999.0
 
 
+def test_detect_vram_preflight_fallback():
+    """Falls back to preflight total VRAM when cf-orch is unreachable."""
+    # Build a fake scripts.preflight module with get_gpus returning two GPUs.
+    fake_scripts = ModuleType("scripts")
+    fake_preflight = ModuleType("scripts.preflight")
+    fake_preflight.get_gpus = lambda: [  # type: ignore[attr-defined]
+        {"vram_total_gb": 8.0},
+        {"vram_total_gb": 4.0},
+    ]
+    fake_scripts.preflight = fake_preflight  # type: ignore[attr-defined]
+
+    with patch("circuitforge_core.tasks.scheduler.httpx") as mock_httpx, \
+         patch.dict(
+             __import__("sys").modules,
+             {"scripts": fake_scripts, "scripts.preflight": fake_preflight},
+         ):
+        mock_httpx.get.side_effect = ConnectionRefusedError()
+        result = detect_available_vram_gb()
+
+    assert result == pytest.approx(12.0)  # 8.0 + 4.0 GB
+
+
 # ── TaskScheduler basic behaviour ─────────────────────────────────────────────
 
 def test_enqueue_returns_true_on_success(tmp_db: Path):