Fix sparsepoly duplicate terms

[woojucy]

Description

This PR reintroduces the changes from the previously closed PR (#915) to address the issue where duplicate terms with the same degree were not explicitly handled in univariate::SparsePolynomial::from_coefficients_vec. In the current implementation, multiple terms with the same degree remain as separate entries, leading to inconsistencies in polynomial operations and unnecessary redundancy.

To resolve this, an is_valid_coefficients_vec function has been introduced to pre-validate the input vector before constructing a sparse polynomial. This ensures that duplicate terms are merged, preventing redundancy.

Changes Introduced

• Implemented is_valid_coefficients_vec to check for duplicate terms.
• Ensured that from_coefficients_vec only processes pre-validated input.
• Added three test cases to verify the correctness of the validation function.
• Considered different approaches for handling rand_sparse_poly behavior:
  • Option 1: Integrate the validity check within rand_sparse_poly and implement a retry mechanism.
  • Option 2: Adjust the output based on is_valid_coefficients_vec and modify related test cases accordingly.

Notes

• The previous PR was automatically closed due to the deletion of the base branch.
• This PR restores those changes while addressing all previous feedback.
• Further discussion is welcome regarding how rand_sparse_poly should handle invalid input.

[z-tech]

This got automatically closed when it was based on a branch that was merged and deleted: #915 (comment)

[z-tech]

@woojucy you need to the merge conflicts otherwise looks good to me

[woojucy]

Merge conflicts resolved! Let me know if anything else needs fixing.