test(model-loaders): add regression tests for checkpoint key-remapping

[Pfannkuchensack]

Summary

This PR fixes a FLUX/Qwen single-file loading regression and adds the regression-test coverage that would have caught it.

The fix. Qwen2_5_VLForConditionalGeneration._checkpoint_conversion_mapping is empty in transformers 5.x, so the if key_mapping: guard skipped remapping and ComfyUI single-file Qwen2.5-VL encoder checkpoints (visual.*, model.*) failed to load: their keys were reported as unexpected while model.visual.* params stayed as meta tensors. We fall back to the legacy mapping when transformers no longer provides one; a negative lookahead keeps new-layout checkpoints untouched.

The tests. Extract the state-dict key surgery in the qwen_image, flux, and anima loaders into pure module-level helpers and cover them with unit tests, so a regression like this one fails in CI instead of at load time.

• qwen_image: extract _remap_qwen_vl_checkpoint_keys
• flux: move the FLUX.2 BFL->diffusers transformer/VAE converters into a new flux2_state_dict_utils module, deduplicating the two identical copies that lived in the checkpoint and GGUF loader classes
• anima: extract _strip_anima_bundle_prefix

Tests use fixtures captured from real single-file checkpoints and, where the diffusers architecture builds cheaply, assert on the meta device that every converted key maps to a real parameter (the FLUX.2 VAE is checked for full bijective coverage). Behavior of all loaders is unchanged.

Related Issues / Discussions

N/A

QA Instructions

Automated (no network, no GPU, no real weights):
uv run --extra cuda --extra test python -m pytest tests/backend/model_manager/load/ --no-cov

• 28 new tests pass; the full load/ suite stays green (246 passed, 122 skipped).
• Regression counter-check: temporarily remove the fallback mapping in _remap_qwen_vl_checkpoint_keys → test_qwen_image_state_dict_utils.py goes red (incl. the bug-report-keys test), proving the tests catch the original bug.

Manual (optional): load a ComfyUI single-file Qwen2.5-VL encoder (e.g. qwen_2.5_vl_7b_fp8_scaled.safetensors) and run a qwen_image_text_encoder invocation — before the fix it raised Failed to load all parameters from checkpoint. Meta tensors remain: [...] with a 728 unexpected keys warning; after the fix it loads cleanly.

Merge Plan

Standard merge — no DB/schema changes, no migrations. Loader behavior is unchanged (converters were only extracted/deduplicated), validated end-to-end against real FLUX.2 transformer/VAE checkpoints (0 unmatched keys).

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[JPPhoto]

LGTM!