[Auto] Add mistral regex regression test (cluster-45081-3): merged 1 of 2 PRs

[evalstate]

Cluster: cluster-45081-3
Base: origin/main

Merged PRs:

• Fix AttributeError in _patch_mistral_regex when fix_mistral_regex=True  huggingface/transformers#45317: Merged cleanly; code fix was already present on main, but this PR still adds the reviewed regression test for the mistral regex patch path.

Skipped PRs:

• fix AttributeError in _patch_mistral_regex huggingface/transformers#45086: Skipped as duplicate/no-op; its 3-line tokenizer fix is already present on main and a test merge produced no tracked changes.

Failed PRs:

• None.

Notes:

• Verified and switched to the cluster branch before triage.
• Cluster contains issue _patch_mistral_regex crashes with AttributeError: 'tokenizers.Tokenizer' object has no attribute 'backend_tokenizer' when loading Mistral tokenizer with fix_mistral_regex=True huggingface/transformers#45081 and PRs fix AttributeError in _patch_mistral_regex huggingface/transformers#45086 and Fix AttributeError in _patch_mistral_regex when fix_mistral_regex=True  huggingface/transformers#45317; issue is still open.
• Fetched PR heads locally and used local-only git merge operations.
• After merging Fix AttributeError in _patch_mistral_regex when fix_mistral_regex=True  huggingface/transformers#45317, the only tracked diff vs origin/main is tests/models/auto/test_tokenization_auto.py with the added regression test.
• A trial merge of fix AttributeError in _patch_mistral_regex huggingface/transformers#45086 with --no-commit applied cleanly but resulted in no staged tracked changes, so it was aborted and classified as duplicate.
• Attempted targeted pytest for mistral patching tests, but the local environment lacked required Python deps: first transformers import path, then numpy.

Next steps:

• If validation is needed, run the targeted tokenizer tests in an environment with project dependencies installed.
• If preparing a human-submitted PR, document that fix AttributeError in _patch_mistral_regex huggingface/transformers#45086 was checked and skipped as duplicate while Fix AttributeError in _patch_mistral_regex when fix_mistral_regex=True  huggingface/transformers#45317 contributed only the regression test on top of main.
• Keep the branch as-is if the goal is to preserve the merged regression test from Fix AttributeError in _patch_mistral_regex when fix_mistral_regex=True  huggingface/transformers#45317.