`HTMLParser.unknown_decl` receives corrupted data (`'CDATA['`) when parsing an empty `CDATA` section

Bug report

Bug description:

When parsing the input <![CDATA[]]>, the unknown_decl hook is incorrectly called with the corrupted, partial string 'CDATA['.

A correct parser has only two possible-and-correct behaviors:

• If CDATA is supported: Call handle_cdata('').
• If the declaration is "unrecognized," the unknown_decl hook must receive the entire content inside <!...>, which would be '[CDATA[]]'.

The actual result ('CDATA[') matches neither of those. I use the private _set_support_cdata(True) method here since I think it was the only available trigger to activate this specific code path to expose the bug.

from html.parser import HTMLParser

class CdataBugParser(HTMLParser):
    def __init__(self):
        super().__init__()
        self.unknown_decls = []

    def unknown_decl(self, data):
        self.unknown_decls.append(data)

html_input = "<![CDATA[]]>"
parser = CdataBugParser()
parser._set_support_cdata(True)
parser.feed(html_input)
print(parser.unknown_decls)

['CDATA[']

CPython versions tested on:

3.12

Operating systems tested on:

Linux

まず、HTMLParser を使って issue の Python 再現コードを実行し、unknown_decl を呼び出す宣言処理の経路を調査します。空の CDATA セクションのカバレッジを追加し、パースの結果が 'CDATA[' ではなく、handle_cdata('') または完全な未知の宣言 '[CDATA[]]' のいずれかになることを確認します。