Fix mypy errors in batch 3 part 1 (#1282)

[rosspeili]

Annotate Tensor optional fields and add guards in _namedtensor. Default beta in compute_thc_resources when None (same pattern as DF). Return empty FermionOperator when plane_wave external potential has no terms.

[gemini-code-assist]

Code Review

This pull request introduces several type safety improvements, bug fixes, and resource estimation updates. In _namedtensor.py, type annotations were added to the Tensor class, and safety checks were introduced to raise TypeError when required attributes are missing. The vectorize method was also updated to use Literal types and direct array reshaping. In plane_wave_hamiltonian.py, a fallback was added to return an empty FermionOperator if the operator is None. Lastly, in compute_thc_resources.py, a fallback calculation for beta was implemented when it is not provided, and float division was replaced with integer division. There are no review comments, so we have no feedback to provide.

[rosspeili]

@mhucka I moved to batch 3 from #1282, this PR fixes strict-mypy issues in _namedtensor, compute_thc_resources (default beta when None, same as DF), and a plane_wave_hamiltonian return narrowing leftover from #1365. Let me know how this looks and can move to next part.