injection.combined merges shell_fragment nodes from ALL RUN commands into
a single bash parse tree. The bash parser receives fragments from different
RUN blocks adjacently (no separator bytes between them), causing it to
parse the tail of one RUN command and the head of the next as a single
command node. This makes arguments in subsequent RUN commands receive
@variable.parameter highlights, causing visible color bleed.
Fix: remove injection.combined so each shell_fragment gets its own bash
tree. Continuation lines ending in && produce has_error=true trees, but
this is harmless for highlighting in practice.
Fixes#6530, #7013, #7062, #6975, #5605
