fix(multimodal): gate the Qwen2-VL prefill export on the feature that calls it

[inureyes]

What breaks

Pipeline Parallel CI / 2-host logical fails on every PR that touches the pipeline-parallel paths. The pipeline-parallel step itself passes; only the trailing Clippy on PP modules step fails, and it fails on src/multimodal/, which such a PR typically does not touch at all.

export_qwen2_vl_prefill is imported at src/multimodal/host_preprocessor.rs and defined at src/multimodal/host_preprocessor_export.rs unconditionally, but its only non-test call site is inside a #[cfg(feature = "xla-iree")] block. The workflow runs cargo clippy -p mlxcel --lib --tests -- -D warnings with no features, so that call site compiles out, the import becomes unused, the function becomes dead, and -D warnings promotes both to errors.

It went unnoticed because the workflow is path-filtered to the pipeline-parallel modules and last ran on 2026-07-20, while the unconditional import arrived on 2026-07-24 in edc0ebbb6. The first PR to touch those paths since is where it surfaced.

The fix

Gate the import and the definition on any(feature = "xla-iree", test). The feature arm matches the single production call site; the test arm is needed because the module tests exercise the function irrespective of features, and dropping them would trade a lint failure for lost coverage.

Validation

Reproduced the exact CI invocation locally. Before: two errors, unused import: export_qwen2_vl_prefill and function export_qwen2_vl_prefill is never used, and could not compile mlxcel (lib). After: Finished dev profile, clean.

The normal development configuration is unaffected: cargo clippy --release --lib --tests --features metal,accelerate is clean, and the Qwen2-VL export tests still compile and pass. One unrelated test in that module, xla_loader_keeps_text_and_unqualified_vlm_image_capability_false, fails both before and after this change and also fails on main, so it is out of scope here.

Why this is filed as its own PR

It is a repository-wide unblock rather than part of any feature branch, and it is the second breakage of this shape found this week after the dtolnay/rust-toolchain pin in #954: pre-existing on main, invisible because a path filter or a feature gate kept any run from exercising the failing combination. The nightly verification job added in #953 targets exactly that blind spot.