bindings/c#: reject out-of-range nbits in mul_n_aggregate

[arib06]

Pairing.mul_n_aggregate takes the randomization bit-length as a signed int and forwards it to the native blst_pairing_mul_n_aggregate_pk_in_g{1,2} as (size_t)nbits with no range check. A negative nbits becomes SIZE_MAX, and any nbits greater than scalar.Length*8 makes POINTonE{1,2}_mult_w5 read the scalar as a wider value than the caller supplied, so the native side reads past the end of the scalar buffer (the C core only guards nbits != 0). The Go binding already rejects this (randBits < 0 || randBits > 256) and Rust validates scalar.len() against nbits; the C# binding was the one left unchecked.

Both overloads now reject an nbits that does not fit the scalar before the native call.

Reproduced against the native path with AddressSanitizer (nbits = 256, 1-byte scalar):

ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1 ... 30 bytes after a 1-byte region
    #0 POINTonE2_mult_w5 e2.c:443
    #1 PAIRING_Aggregate_PK_in_G1 aggregate.c:281
    #2 blst_pairing_mul_n_aggregate_pk_in_g1 aggregate.c:359

A 32-byte scalar with nbits = 256 reads in bounds and is unaffected. poc.cs gets a regression case that fails without the guard and passes with it; dotnet run -c Release is green.

[dot-asm]

As it says in the beginning of the modified .cs file "DO NOT EDIT THIS FILE." :-) Anyway, I've moved the tweaked modifications to the right place. Thanks!