Fix kagglehub fake API server (#1522)

Starting with `kagglehub` 0.4.0, a new generated Python client is used
which sends POST requests instead of GET. The endpoint is now controlled
via the `KAGGLE_API_ENVIRONMENT` env variable.

http://b/475292408

Author: rosbo

tests/utils/kagglehub.py

@@ -1,3 +1,4 @@
+import json
 import os
 import threading
 import re
@@ -7,6 +8,8 @@
 from test.support.os_helper import EnvironmentVarGuard
 from http.server import BaseHTTPRequestHandler, HTTPServer
 
+from kagglesdk.kaggle_env import get_endpoint, get_env
+
 class KaggleAPIHandler(BaseHTTPRequestHandler):
     """
     Fake Kaggle API server supporting the download endpoint.
@@ -15,15 +18,18 @@ class KaggleAPIHandler(BaseHTTPRequestHandler):
     def do_HEAD(self):
         self.send_response(200)
 
-    def do_GET(self):
-        m = re.match("^/api/v1/models/(.+)/download/(.+)$", self.path)
-        if not m:
+    def do_POST(self):
+        content_length = int(self.headers.get('Content-Length', 0))
+        body_bytes = self.rfile.read(content_length)
+        request_body = json.loads(body_bytes.decode('utf-8'))
+
+        if self.path != "/api/v1/models.ModelApiService/DownloadModelInstanceVersion":
             self.send_response(404)
             self.wfile.write(bytes(f"Unhandled path: {self.path}", "utf-8"))
             return
 
-        model_handle = m.group(1)
-        path = m.group(2)
+        model_handle = f"{request_body["ownerSlug"]}/{request_body["modelSlug"]}/keras/{request_body["instanceSlug"]}/{request_body["versionNumber"]}"
+        path = request_body["path"]
         filepath = f"/input/tests/data/kagglehub/models/{model_handle}/{path}"
         if not os.path.isfile(filepath):
             self.send_error(404, "Internet is disabled in our tests "
@@ -41,14 +47,12 @@ def do_GET(self):
 
 @contextmanager
 def create_test_kagglehub_server():
-    endpoint = 'http://localhost:7777'
     env = EnvironmentVarGuard()
-    env.set('KAGGLE_API_ENDPOINT', endpoint)
-    test_server_address = urlparse(endpoint)
+    env.set('KAGGLE_API_ENVIRONMENT', 'TEST')
     with env:
-        if not test_server_address.hostname or not test_server_address.port:
-            msg = f"Invalid test server address: {endpoint}. You must specify a hostname & port"
-            raise ValueError(msg)
+        endpoint = get_endpoint(get_env())
+        test_server_address = urlparse(endpoint)
+
         with HTTPServer((test_server_address.hostname, test_server_address.port), KaggleAPIHandler) as httpd:
             threading.Thread(target=httpd.serve_forever).start()