fix(markup): fix invalid closing tag parsing

[TomJGooding]

Fix the markup parsing of invalid closing tags missing the closing square bracket.

Currently [/ is interpreted as an auto-closing tag, which will crash with a MarkupError if there's nothing to close.

Fixes #6155

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[TomJGooding]

This handles the unexpected EOF when parsing "open_closing_tag". I wasn't sure if we want to handle other invalid tags similar to the "open_tag" parsing?

textual/src/textual/markup.py

Lines 375 to 389 in a77700c

	elif token_name == "open_tag":
	tag_text = []
	eof = False
	contains_text = False
	for token in iter_tokens:
	if token.name == "end_tag":
	break
	elif token.name == "text":
	contains_text = True
	elif token.name == "eof":
	eof = True
	tag_text.append(token.value)
	if contains_text or eof:
	# "tag" was unparsable