Fix Mistral SUT model ID formatting

[kaulastudies]

Summary

Fixes Mistral SUT model ID formatting by separating ModelBench's canonical SUT identity from the model ID sent to Mistral's API.

Dynamic Mistral definitions keep the canonical ModelBench UID:

mistral/<model>:mistral

but MistralSUTFactory.make_sut() now sends only the model component (<model>) to Mistral's model_info() lookup and to MistralAISut.

This prevents native Mistral model IDs such as mistral-medium-3 from being rewritten to mistral/mistral-medium-3, which returns 404 from the Mistral API.

Tests

The regression test verifies both sides explicitly:

• ModelBench UID still preserves the maker: foo/bar:mistral
• Mistral API/runtime model ID is only bar
• model_info() is called with bar

Validation

• Mistral SUT factory tests: 3 passed
• Related SUT factory tests: 13 passed
• Black: passed
• MyPy: passed
• git diff --check: passed

Closes #1659

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