Fix Mobius test-model export and llama.cpp discrepancy checks

[Tommaso Adani (tadani3)]

Describe your changes

Fix olive run --test for workflows using MobiusBuilder.

• Export the saved reduced test model instead of the original Hugging Face model.
• Honor the two-layer model generated for discrepancy checks, avoiding full-model downloads and exports.
• Materialize test weights when the test-model directory contains only configuration.
• Raise a clear error when test_model_config is set without test_model_path.
• Preserve the original Hugging Face model ID for normal exports.
• Forward --test_llama_path to enable llama.cpp conversion and benchmarking.
• Add regression tests for both fixes.

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary. No documentation changes required.
• Lint and apply fixes to your code by running lintrunner -a.
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.

Release note: Fixed olive run --test to honor reduced two-layer models used by discrepancy checks when exporting with Mobius, and to honor --test_llama_path for llama.cpp benchmarking.

(Optional) Issue link

N/A

[Copilot]

Pull request overview

This PR fixes olive run --test workflows that export models with MobiusBuilder by ensuring the export path honors the reduced “test model” directory (including the two-layer model used for discrepancy checks) and by wiring --test_llama_path through so llama.cpp benchmarking/conversion can be enabled in test runs.

Changes:

• Updated MobiusBuilder to export from the persisted test_model_path (and materialize missing test weights when the directory is config-only), while preserving the original Hugging Face model ID for normal exports.
• Updated olive run to forward --test_llama_path into discrepancy-check pass injection.
• Added regression tests covering Mobius test-model export behavior and CLI forwarding of llama.cpp settings.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/passes/onnx/test_mobius_model_builder.py	Adds regression tests ensuring Mobius uses the saved test-model directory, materializes missing weights, and errors when misconfigured.
test/cli/test_cli.py	Extends CLI test coverage to assert --test_llama_path is forwarded into injected passes/config.
olive/passes/onnx/mobius_model_builder.py	Switches Mobius export to use the reduced test-model directory when test_model_config is active, with validation and weight materialization.
olive/cli/run.py	For olive run --test, forwards --test_llama_path to add_discrepancy_check_pass.

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