Refactor all random samplers to use `Distribution`

Agreed, we have to refactor all random samplers to use `Distribution`-s. This is done in some places but not consistently.

_Originally posted by @Al-Kindi-0 in https://github.com/0xMiden/crypto/pull/720#discussion_r2635009578_

Leftover from 0xMiden/crypto#720

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**Clarified scope** ([context](https://github.com/0xMiden/crypto/pull/753#issuecomment-3717821401)): this is about test helpers, not key generation.

In scope:
- Messages to sign
- Polynomials (Falcon)
- Plaintexts for encryption

Out of scope:
- SecretKey/Nonce generation for DSA, AEAD, ECDH modules

Original thread: [PR 0xMiden/crypto#720 comment](https://github.com/0xMiden/crypto/pull/720#discussion_r2634577006)