fix: accept tuple batches in rerank_pairs

[xblwh]

rerank_pairs() accepts an iterable of query/document pairs, but currently wraps every tuple as a single pair. Passing ((query, doc1), (query, doc2)), a one-element tuple, or an empty tuple raises a tokenizer TypeError.

Only wrap a tuple containing two strings as the existing single-pair shorthand. Treat tuple batches like lists when choosing the small-batch path. The regression tests cover 0–3 pairs with sequential and parallel execution, plus the existing single-pair behavior, using the real MiniLM reranker and canonical scores.

Validation

• The eight tuple-batch cases fail on unchanged main; both single-pair cases already pass.
• pytest tests/test_text_cross_encoder.py tests/test_common.py -q: 19 passed, with both current compatible dependencies and the Python 3.12 versions selected from poetry.lock.
• With the lockfile versions: mypy fastembed --disallow-incomplete-defs --disallow-untyped-defs --disable-error-code=import-untyped passes for all 64 source files; pyright tests/type_stub.py and ruff check fastembed tests pass.
• The repository's pre-commit hooks pass for both changed files; git diff --check passes.

The full model suite and other OS/Python combinations were not run. Broader Ruff checks report the same existing formatting and experiment-notebook lint findings on unchanged main; resolving the newest dependencies also exposes five existing mypy errors outside this change. These are unchanged by this patch.

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ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 90fb066a-825d-40db-be60-35c5d79de0e8

📥 Commits

Reviewing files that changed from the base of the PR and between 0dab99c and 0d07193.

📒 Files selected for processing (2)

• fastembed/rerank/cross_encoder/onnx_text_model.py
• tests/test_text_cross_encoder.py

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📝 Walkthrough

Walkthrough

_rerank_pairs now treats a two-string tuple as one pair and accepts tuples of pairs for batch processing. Tests cover empty, single, and multiple tuple-of-pairs inputs with sequential and parallel execution. Tests also verify direct single-pair tuples.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 0d071

The tuple-batch behavior is covered across supported input sizes and execution modes, with no established current-head defect.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the tuple-batch fix, preserved single-pair behavior, regression tests, and validation results. It is directly related to the changeset.
Title check	✅ Passed	The title clearly and concisely identifies the main change: accepting tuple batches in rerank_pairs.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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