Fix shadowed variable in getRandomPolynomial to correctly disable blinding (PLONK bn254)

[viktorking7]

Description

• Summary: Replace a shadowed assignment that prevented creating a zero polynomial when n == -1.
• Problem: In getRandomPolynomial, a was redeclared with a := under n == -1, leaving the outer slice nil and failing to cancel blinding as intended.
• Fix: Change a := make(... to a = make(... so the outer variable is set.
• Impact: Ensures correct “cancel the blinding” behavior.

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Note

Fixes shadowed assignment in getRandomPolynomial so n == -1 yields a zero polynomial across all PLONK backends and the template.

• PLONK backends:
  • Update getRandomPolynomial in backend/plonk/{bls12-377,bls12-381,bls24-315,bls24-317,bn254,bw6-633,bw6-761}/prove.go to assign a = make(...) (not a := ...) when n == -1, ensuring a zero polynomial and disabling blinding.
• Code generation template:
  • Apply the same fix in internal/generator/backend/template/zkpschemes/plonk/plonk.prove.go.tmpl.

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