Server isolation and detailed errors

Isolated the JsonServer to server.py inside protocols/json

Added specific errors for server startup timeout and server startup
failed

Refactored errors for Core to CoreError generic and refactored
StateError to CoreStateMismatchError derived from CoreError

test/common.py

@@ -0,0 +1,27 @@
+import socket
+
+
+def find_available_port(
+    preferred_port: int | None = None,
+    host: str = "127.0.0.1",
+) -> int:
+    """Return an available TCP port on the given host.
+
+    The preferred port is returned when available. Otherwise, the operating
+    system selects an available ephemeral port.
+    """
+    if preferred_port is not None:
+        if not 1 <= preferred_port <= 65535:
+            raise ValueError("preferred_port must be between 1 and 65535")
+
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+            try:
+                sock.bind((host, preferred_port))
+            except OSError:
+                pass
+            else:
+                return preferred_port
+
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+        sock.bind((host, 0))
+        return int(sock.getsockname()[1])