fix: enforce poseidon2 encryption nonce bounds

[Forostovec]

Reason:
Previously poseidon2 encryption packed (encryption_nonce, L) into a single field element assuming a 128-bit nonce, but this was only documented in a TODO comment and never enforced. Callers could pass arbitrary field elements as nonces, breaking the intended 128-bit nonce invariant and making the encoding less clearly injective from a security/analysis point of view.

Overview of changes:

• Add an assertion in poseidon2_encrypt to ensure encryption_nonce < 2^128 (TWO_POW_128).
• Add the same assertion in poseidon2_decrypt to mirror the invariant at decryption time and keep both sides in sync with the documented design.
• Remove the obsolete TODO that referenced these missing checks.