Install YOLO26 deps in model CI

[JacobSzwejbka]

Summary:

Install the YOLO26 model requirements in the generic test_model.sh path before exporting the model. The OSS test-models-linux yolo26 portable job imports examples.models.yolo26 through examples.portable.scripts.export, and that import requires ultralytics.

Use the same --upgrade-strategy only-if-needed install style as .ci/scripts/test_yolo26.sh to avoid replacing the CI torch install.

Test Plan:

bash -n .ci/scripts/test_model.sh
git diff --check
PYTHON_EXECUTABLE=true bash .ci/scripts/test_model.sh yolo26 cmake none
lintrunner -r origin/main

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20780

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❌ 3 New Failures

As of commit 39c9fc9 with merge base 69eb495 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_vit_model
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t a712a5d5ea14138a4355c149d8baba5085f8c46cd67eaaa8d3e28ccc06b9f163 /exec failed with exit code 1
• trunk / test-models-macos-coreml (mobilebert) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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