server : add support for external server for tests (#16243)

This commit adds support for using an externally started llama-server
instance for the server tests. This can be enabled by setting the
DEBUG_EXTERNAL environment variable.

The motivation for this is to allow debugging of the server itself
when investigating a test failure. Instructions for how to do this are
added to the README.md file in the tests directory.

tools/server/tests/utils.py

@@ -99,8 +99,12 @@ class ServerProcess:
             self.debug = True
         if "PORT" in os.environ:
             self.server_port = int(os.environ["PORT"])
+        self.external_server = "DEBUG_EXTERNAL" in os.environ
 
     def start(self, timeout_seconds: int | None = DEFAULT_HTTP_TIMEOUT) -> None:
+        if self.external_server:
+            print(f"[external_server]: Assuming external server running on {self.server_host}:{self.server_port}")
+            return
         if self.server_path is not None:
             server_path = self.server_path
         elif "LLAMA_SERVER_BIN_PATH" in os.environ:
@@ -244,6 +248,9 @@ class ServerProcess:
         raise TimeoutError(f"Server did not start within {timeout_seconds} seconds")
 
     def stop(self) -> None:
+        if self.external_server:
+            print("[external_server]: Not stopping external server")
+            return
         if self in server_instances:
             server_instances.remove(self)
         if self.process: