test(minimax): use 'content' key so compress_history_tags actually runs

[shaun0927]

Problem

tests/test_minimax.py::TestMiniMaxCompressHistoryTags::test_think_tag_compressed_in_old_messages
constructs its message fixture with the key \"prompt\":

messages = [
    {"role": "assistant", "prompt": f"<think>{long_think}</think>\nShort answer."},
    {"role": "user", "prompt": "Follow up"},
] + [{"role": "user", "prompt": f"msg{i}"} for i in range(12)]

But compress_history_tags in llmcore.py reads msg['content']:

# llmcore.py:38
c = msg['content']

So the function raises KeyError: 'content' on the first iteration of the
compression loop. The assertion line that the test was written to exercise
is never reached.

$ pytest tests/test_minimax.py::TestMiniMaxCompressHistoryTags -v
E   KeyError: 'content'
llmcore.py:38: KeyError
FAILED tests/test_minimax.py::TestMiniMaxCompressHistoryTags::test_think_tag_compressed_in_old_messages

Fix

Replace \"prompt\" with \"content\" in the fixture and in the
post-compression lookup so the test actually runs the compression
code path it claims to verify.

         messages = [
-            {"role": "assistant", "prompt": f"<think>{long_think}</think>\nShort answer."},
-            {"role": "user", "prompt": "Follow up"},
-        ] + [{"role": "user", "prompt": f"msg{i}"} for i in range(12)]
+            {"role": "assistant", "content": f"<think>{long_think}</think>\nShort answer."},
+            {"role": "user", "content": "Follow up"},
+        ] + [{"role": "user", "content": f"msg{i}"} for i in range(12)]
         ...
-        first_content = result[0]["prompt"]
+        first_content = result[0]["content"]

Verification

$ pytest tests/test_minimax.py::TestMiniMaxCompressHistoryTags -v
PASSED

The test now exercises the real <think> tag truncation path against
compress_history_tags, which is what the test name promises.