Do not mistake a bracketed object name for a replacemsg placeholder

[ggiesen]

Fixes #908.

_parse_out_offending() moves a config system replacemsg buffer aside and leaves a ["<name>"] placeholder, which _build_nested_config() restores. The restore was guarded by the presence of a bracket alone, so any line containing one was taken for a placeholder, and a lookup miss raised ValueError: Input line is malformed. out of __init__, losing the whole config.

_build_nested_config() receives the first indented line of a top-level block, so an object name with a bracket there is enough to trigger it:

config firewall address
    edit "obj[1]"
        set subnet 10.0.0.0 255.0.0.0
    next
end

The guard now also requires the quoted key to be one that _get_uncommon_lines() produced, and leaves the line alone otherwise.

Tests cover both the bracketed name and a genuine placeholder still being restored. 823 passed, 6 skipped; ruff, pylint and mypy clean.