Skills test: set explicit max_tokens=4096 instead of 0

max_tokens=0 made stream_agent_loop omit the param entirely, which
on some OpenAI-compat upstreams (DeepSeek in the report) meant the
model defaulted to a very short or zero-token completion — the user
saw "the model returned an empty" even though normal chat with the
same model worked (chat sends its preset's max_tokens). Match the
chat default.

routes/skills_routes.py

@@ -691,8 +691,12 @@ async def _run_skill_test_once(md: str, task: str, url, model, headers, owner) -
         {"role": "user", "content": task},
     ]
     try:
+        # max_tokens explicitly set: passing 0 lets some upstreams (Ollama,
+        # OpenAI-compat) generate an empty completion, which manifested as
+        # the skill test returning nothing while chat (which carries its
+        # preset's max_tokens) worked. 4096 matches the chat default.
         async for chunk in stream_agent_loop(url, model, messages, headers=headers,
-                                             temperature=0.3, max_tokens=0, max_rounds=8, owner=owner):
+                                             temperature=0.3, max_tokens=4096, max_rounds=8, owner=owner):
             if not chunk.startswith("data: ") or chunk.strip() == "data: [DONE]":
                 continue
             try: