Fix YUAN2 conversation template stripping message characters

[JSap0914]

Why are these changes needed?

The YUAN2 separator style in fastchat/conversation.py joins messages with
the literal <n> separator and then calls ret.rstrip("<n>") to drop the
trailing separator before appending seps[0].

str.rstrip treats its argument as a set of characters, not a suffix, so it
removes every trailing <, n, and > character. Any message that ends in one
of those characters gets corrupted. For example:

conv = get_conv_template("yuan2")
conv.append_message(conv.roles[0], "What is the value of n")
conv.append_message(conv.roles[1], None)
conv.get_prompt()
# before: 'What is the value of <sep>'   (trailing "n" lost)
# after:  'What is the value of n<sep>'

The fix removes only the trailing <n> separator with an explicit suffix check,
leaving message content intact.

Related issue number (if applicable)

N/A

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).

Verification (Linux, offline):

$ python3 -m pytest tests/test_conversation.py -v
tests/test_conversation.py::TestYuan2Template::test_message_ending_in_angle_bracket_is_preserved PASSED
tests/test_conversation.py::TestYuan2Template::test_message_ending_in_n_is_preserved PASSED
tests/test_conversation.py::TestYuan2Template::test_separator_between_messages_is_kept PASSED
3 passed

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