xml.dom.minidom: serializing a comment ending in '-' silently produces non-well-formed XML

Bug report

Bug description:

xml.dom.minidom guards comment serialization against -- (illegal inside an XML comment): Comment.writexml raises ValueError if "--" in self.data (test testSerializeCommentNodeWithDoubleHyphen). It misses a comment whose data ends with -: the trailing - fuses with the closing --> into --->, which contains --.

import xml.dom.minidom as m
doc = m.getDOMImplementation().createDocument(None, None, None)
doc.appendChild(doc.createComment("see note-"))
out = doc.toxml()          # '<?xml version="1.0" ?><!--see note--->'
m.parseString(out)         # xml.parsers.expat.ExpatError: not well-formed

minidom silently emits XML it cannot reparse. The existing -- guard already takes the stance that serialization must fail loudly on comment content it cannot represent; the trailing-- case is a gap in that guard.

CPython versions tested on:

CPython main

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-156813

從 xml.dom.minidom 的 Comment.writexml 實作和現有名為 testSerializeCommentNodeWithDoubleHyphen 的測試開始。重現以 '-' 結尾的註解的序列化，並執行 minidom 測試；完成的標準是，結尾帶連字號的無效情況與包含 '--' 的註解一樣被一致拒絕。