fix(bls12_381): added raise for all InvalidParameter exceptions in decode_g1_scalar_pair / decode_g2_scalar_pair

[ImanKalyanChakraborty]

🗒️ Description

Fix missing raise statements in BLS12-381 scalar pair decoders.

Both decode_g1_scalar_pair and decode_g2_scalar_pair were constructing an InvalidParameter exception when input length checks failed, but not raising it. This resulted in the validation being a no-op, allowing execution to continue with invalid inputs.

While current callers (bls12_g1_msm, bls12_g2_msm) enforce correct input lengths and pass properly sized slices, thus making the bug effectively unreachable in existing flows, this behavior breaks the function’s contract and could lead to silent failures if these utilities are used elsewhere (e.g., tests, tooling, or future refactors).

This PR adds the missing raise statements to ensure proper error handling and defensive validation.

🔗 Related Issues or PRs

Fixes #2734

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  just static

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