fix: discover pre-running Docker servers even when docker-compose is unavailable

In CI (GitHub Actions ubuntu-latest / ubuntu-24.04), Baikal runs as a
service container that is already up before the tests start. The legacy
`docker-compose` standalone binary is not installed on these runners
(only the `docker compose` plugin is), so `verify_docker()` returned
False and `_discover_docker_servers()` bailed out without registering
Baikal.  The async test job used `-k baikal` with no fallback, producing
exit-code 5 (no tests collected).

Two fixes:
* `verify_docker()` now accepts either `docker-compose --version` or
  `docker compose version` (plugin form).
* `_discover_docker_servers()` no longer skips registration entirely when
  Docker is not startable; it still registers servers that are already
  accessible (e.g. a CI service container), letting `DockerTestServer.start()`
  handle them with its existing "already running" detection.

prompt: have a look into this failure: gh run view 24923998455

AI Prompts:
claude-sonnet-4-6: have a look into this failure: `gh run view 24923998455`

Author: tobixen

tests/test_servers/base.py

@@ -308,30 +308,20 @@ def verify_docker() -> bool:
         Check if docker and docker-compose are available.
 
         Returns:
-            True if docker-compose is available and docker daemon is running
+            True if docker compose is available and docker daemon is running
         """
         import subprocess
 
-        try:
-            subprocess.run(
-                ["docker-compose", "--version"],
-                capture_output=True,
-                check=True,
-                timeout=5,
-            )
-            subprocess.run(
-                ["docker", "ps"],
-                capture_output=True,
-                check=True,
-                timeout=5,
-            )
-            return True
-        except (
-            subprocess.CalledProcessError,
-            FileNotFoundError,
-            subprocess.TimeoutExpired,
-        ):
-            return False
+        def _run(*cmd: str) -> bool:
+            try:
+                subprocess.run(list(cmd), capture_output=True, check=True, timeout=5)
+                return True
+            except (subprocess.CalledProcessError, FileNotFoundError, subprocess.TimeoutExpired):
+                return False
+
+        # Accept either the legacy standalone binary or the modern plugin form.
+        compose_ok = _run("docker-compose", "--version") or _run("docker", "compose", "version")
+        return compose_ok and _run("docker", "ps")
 
     def start(self) -> None:
         """

tests/test_servers/registry.py

@@ -297,8 +297,7 @@ def _discover_docker_servers(self) -> None:
 
         from .base import DockerTestServer
 
-        if not DockerTestServer.verify_docker():
-            return
+        docker_available = DockerTestServer.verify_docker()
 
         # Look for docker-test-servers directories
         docker_servers_dir = Path(__file__).parent.parent / "docker-test-servers"
@@ -312,7 +311,11 @@ def _discover_docker_servers(self) -> None:
                 server_class = get_server_class(server_name)
 
                 if server_class is not None and server_name not in self._servers:
-                    self.register(server_class({"docker_dir": str(server_dir)}))
+                    server = server_class({"docker_dir": str(server_dir)})
+                    # Register if Docker is available (can start containers) OR if
+                    # the server is already running (e.g. a CI service container).
+                    if docker_available or server.is_accessible():
+                        self.register(server)
 
     def get_caldav_servers_list(self) -> list[dict]:
         """