MAINT-7831: Update viewer_manifest.py to use 'with self.prefix(...):'

Now that LLManifest.prefix() supports use as a context manager:

    with self.prefix(...):
        ...

convert existing calls to that form.

This was an interesting exercise because it surfaced at least two places where
the indentation did not match the self.prefix() nesting, plus another place
where existing code was undented without a self.end_prefix() call. (That last
was an uncaught logic bug.) This underscores the value of using a SINGLE
consistent, idiomatic mechanism to limit the scope of each self.prefix() call.