Describe the bug

When a # fmt: off / # fmt: on block has only comments in it and the statement right after it is outside the lines given to --line-ranges black glues that statement onto the # fmt: on line so it ends up inside the comment

With --fast the code is just gone while black still reports the file as reformatted and exits 0. Without --fast the safety check catches it and you get an internal error instead

To Reproduce

For example take this code in settings.py

# fmt: off
#---------+---------#
#  name   |  value  #
#---------+---------#
# fmt: on
TIMEOUT = 30
retries  =  3

And run it with these arguments

$ black --fast --line-ranges 7-7 settings.py
reformatted settings.py

The file now looks like this

# fmt: off
#---------+---------#
#  name   |  value  #
#---------+---------#
# fmt: onTIMEOUT = 30
retries = 3

so TIMEOUT doesn't exist anymore and import settings; settings.TIMEOUT raises AttributeError

Without --fast the same command gives

error: cannot format settings.py: INTERNAL ERROR: Black ... produced code that is not equivalent to the source

If the block is inside a function or class body (statement after it is return 1 or debug = True) it fails even with --fast with produced invalid code since the body ends up empty

Expected behavior

Only line 7 should change (retries = 3) and everything outside the range should be left as it is

Environment

• Black's version: main (11cff13)
• OS and Python version: Linux (WSL2) / Python 3.14.7

Additional context

From what I can tell _handle_comment_only_fmt_block in comments.py takes the newline after the # fmt: on line out of the next leaf's prefix and the line ranges code then writes the unselected statement back with str(node) which puts both on one line. #5477 added a newline in _str_with_standalone_comments for this kind of thing but only when the comment ends with # fmt: skip so comment only fmt off blocks still hit it

This is also why some lines of the existing fmtonoff.py and fmtskip11.py test cases fail when formatted with --line-ranges

I have a fix with tests ready and can open a PR for it