fix: Do not autofill default_model_filename for Python-runtime PyTorch models

[mattwittwer]

What does the PR do?

Since #484, AutoCompleteBackendFields autofills default_model_filename: "model.pt" for every backend: "pytorch" model with an empty platform — including models using the Python-based runtime (runtime: "model.py"). For those models a sibling model.pt is the weights companion to the model.py class, and the runtime gives default_model_filename top priority, so it calls torch.jit.load() on the weights file and fails at load with PytorchStreamReader failed locating file constants.pkl.

Restores the pre-#484 guard — skip filename autofill when runtime is "model.py" — and extends it to the AOTI platform branch (model.pt2), which the Python runtime cannot load either. Output is unchanged for every other config. #484's stated goal (correct .pt2 selection for torch_aoti) is preserved and now pinned by tests.

Adds AutoCompleteBackendFieldsTest to repo_agent_test (which already compiles model_config_utils.cc): the two Python-runtime regression cases plus six cases pinning existing autofill and version-dir sniffing behavior. The regression cases fail without the fix and pass with it.

Checklist

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• build
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Related PRs:

triton-inference-server/server#8913

Where should the reviewer start?

Test plan:

Test case added here

• CI Pipeline ID:

60326198

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Background

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• closes GitHub issue: #xxx

[greptile-apps]

Greptile Summary

The PR prevents Python-runtime PyTorch models from receiving an incompatible default model filename.

• Skips model.pt and model.pt2 filename autofill when the configured runtime is model.py.
• Preserves existing LibTorch and AOTI autofill behavior for default-runtime models.
• Adds regression coverage for Python runtimes, explicit filenames, and model-layout sniffing.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/model_config_utils.cc	Adds a focused runtime guard to PyTorch default-model-filename autofill while preserving platform selection.
src/test/repo_agent_test.cc	Adds comprehensive unit coverage for the corrected Python-runtime behavior and existing autofill paths.

_{Reviews (3): Last reviewed commit: "update use_autofill_filename name" | Re-trigger Greptile}