Fix missing default class name when attaching a spec without prefix or suffix

[discobot]

Fixes #2707.

The attach paths copy the child's root default tree and only apply
mjCDef::NameSpace, which prepends the prefix/suffix to each name. With an
empty prefix and suffix the copy keeps the reserved name main, and
mjCModel::operator+=(mjCDef&) inserted it without a collision check, silently
overwriting def_map["main"]. Since the XML writer omits the class attribute
for any default named main and resolves every element's class through
def_map, this both emits the nameless nested <default/> and corrupts how the
parent's own elements are serialized.

This change renames the incoming default tree's root to a unique name
(main_1, ...) in mjCModel::operator+=(mjCDef&) before insertion, mirroring
the disambiguation that non-empty prefixes already provide. It also covers the
more general collision — e.g. parent and child both defining a class with the
same name, or attaching the same spec twice with identical prefixes — which
previously serialized as duplicate class names that fail to reload with
"repeated default class name".

Added a regression test that attaches a child spec with defaults using empty
prefix/suffix and checks that the parent's global default is not clobbered, the
saved XML loads back, and the reloaded model is identical. Before the fix it
fails with XML Error: empty class name; with the fix the user and XML test
suites pass.

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