fix(lora): correctly detect flattened layer keys in LayerPatcher

[Pfannkuchensack]

Summary

The patcher decided whether a patch's layer keys were flattened (legacy underscore-joined) or real dotted module paths by inspecting only the first key. For FLUX.2 Klein diffusers LoRAs whose first converted layer is a dotless top-level module (e.g. context_embedder), the whole patch was misclassified as flattened, causing assert "." not in layer_key to fail on subsequent dotted keys and crashing LoRA application.

Inspect all keys instead: a flattened key never contains a dot, so the patch is flattened only if no key contains one. Add a regression test covering the mixed dotless/dotted key ordering.

Related Issues / Discussions

https://discord.com/channels/1020123559063990373/1149510134058471514/1517709403858669708

QA Instructions

A FLUX.2 Klein diffusers-format LoRA whose first converted layer targets a dotless top-level module (e.g. transformer.context_embedder, transformer.x_embedder, transformer.proj_out) previously crashed on load with:
File ".../invokeai/backend/patches/layer_patcher.py", line 289, in _get_submodule
assert "." not in layer_key

To verify:

1. Load a FLUX.2 Klein model and apply such a LoRA during denoising — it should now load and apply without the assertion error.
2. Run the patcher unit tests:
   uv run --extra cuda --extra test python -m pytest tests/backend/patches/test_layer_patcher.py -q
   All tests pass (including the new test_apply_smart_model_patches_mixed_dotted_and_dotless_keys regression test, which fails on the old single-first-key heuristic).
3. FLUX.1 LoRAs and other formats are unaffected (their keys already contained dots, so flattened detection is unchanged).

Merge Plan

Standard merge — isolated, low-risk change to LoRA patching logic. No DB/schema changes.

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)