feat: make max length argument explicit

[stephantul]

The max length argument between StaticModel and the trainable counterparts was inconsistent: for StaticModel, we use an unset sentinel to mean "do what the model does" and None to mean no max_length. For trainable models, it was impossible to unset the max length.

[greptile-apps]

Confidence Score: 4/5

[Medium risk] Changes how max length parameter defaults are handled across training classes.

The PR appears safe to merge; an end-to-end truncation test would provide useful non-blocking coverage.

_{Reviews (1) · Last reviewed commit: "feat: make max length argument explicit"}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
model2vec/train/base.py	99.56% <100.00%> (+<0.01%)	⬆️
model2vec/train/classifier.py	100.00% <100.00%> (ø)
model2vec/train/pairs.py	100.00% <100.00%> (ø)
model2vec/train/similarity.py	100.00% <100.00%> (ø)

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