Fix elision of long collections in assertion messages with -vv (#14748)

The message part of assert cond, msg went through saferepr(obj, maxsize), where -vv maps to maxsize=None. That lifts the overall string-length cap, but SafeRepr still inherits reprlib’s per-container limits (maxlist=6 etc.), so e.g. assert False, dir("") stayed elided with ... at any verbosity — the exact reproducer from issue #12307, which #12662 only partially fixed (it covered long scalar reprs but not collections).

Use the _saferepr helper instead, as originally proposed in #12322: it routes maxsize=None to saferepr_unlimited(), which uses a plain repr(), matching how asserted expressions are already displayed at high verbosity.

Closes #12322 Fixes #12307