fix(tests): add 5-second timeout to docker_available() to prevent hang on KVM runner

On the self-hosted KVM runner Docker is on PATH but the daemon socket
is unreachable (firewalled/dropped). subprocess.run(["docker", "info"])
with no timeout hangs indefinitely on a dropped connection, stalling the
coverage job for hours — one hang per @skip_unless_docker()-decorated
class, ~8 per integration suite run.

Add timeout=5 with a TimeoutExpired → False fallback so the check
resolves quickly to "unreachable" rather than blocking.

tests/_docker.py

@@ -10,15 +10,19 @@ import unittest
 def docker_available() -> bool:
     if shutil.which("docker") is None:
         return False
-    return (
-        subprocess.run(
-            ["docker", "info"],
-            stdout=subprocess.DEVNULL,
-            stderr=subprocess.DEVNULL,
-            check=False,
-        ).returncode
-        == 0
-    )
+    try:
+        return (
+            subprocess.run(
+                ["docker", "info"],
+                stdout=subprocess.DEVNULL,
+                stderr=subprocess.DEVNULL,
+                check=False,
+                timeout=5,
+            ).returncode
+            == 0
+        )
+    except subprocess.TimeoutExpired:
+        return False
 
 
 def skip_unless_docker(reason: str = "docker unreachable"):