LTX-2 masks its text conditioning. The model loaded cleanly and died at the
first denoising step:

    ValueError: `attn_mask` is not supported for sage attention

so `auto` choosing sage there is not a slower path, it is a broken one.

The first version of this guard moved it to flash instead, and it failed
again with the same sentence and a different name:

    ValueError: `attn_mask` is not supported for flash-attn 2.

NEITHER fast backend accepts a mask, so falling back from one to the other is
not a fix. For these families the only working backend is the default SDPA,
and the guard now keeps it.

An operator who names sage or flash explicitly is still obeyed -- the override
exists so they can force a backend, and silently ignoring it would be a
different kind of wrong from silently choosing a broken one. They get the
error, which is theirs to get.

Wan and the LTX 0.9 family are unaffected and still take the fast backends;
tests pin both directions.

Suite: 15 failed, 5258 passed -- the same 15 as the baseline.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnmWyKjWZerwsb27jZ