Add type hints to Hubbard constructors

[adminlip]

This adds explicit annotations to the public Fermi- and Bose-Hubbard constructor signatures, covering the dimensions, scalar model parameters, boolean options, and concrete operator return types. The change keeps the implementations unchanged while making this Hamiltonian module easier to type-check incrementally as part of the broader type-hinting effort. Verified with python3 -m compileall -q src/openfermion/hamiltonians/hubbard.py, targeted signature assertions, and git diff --check.

Part of #633

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[gemini-code-assist]

Code Review

This pull request adds type hints to the fermi_hubbard and bose_hubbard functions in src/openfermion/hamiltonians/hubbard.py. These hints specify the expected types for dimensions, physical constants like tunneling and interaction terms, and boolean flags, while also defining the return types as FermionOperator and BosonOperator. No review comments were provided for these changes, and I have no further feedback.

[mhucka]

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