solana-client-tools: add getProgramAccounts fetch helper for zero-copy accounts

[karl-dz]

Target repo

• doublezerofoundation/doublezero-offchain: solana-client-tools crate

Summary

The solana-client-tools crate has a family of helpers for fetching and deserializing zero-copy accounts (T: Pod + PrecomputedDiscriminator):

• try_fetch_zero_copy_data (single account by pubkey)
• try_fetch_zero_copy_data_with_commitment
• try_fetch_multiple_zero_copy_data (multiple accounts by pubkey)

There is no equivalent for fetching every account of a given type from a program (a getProgramAccounts scan filtered by discriminator). Callers that need all accounts of a type currently hand-roll the same loop each time: a getProgramAccounts request with a discriminator memcmp at offset 0, a skip of zero-lamport (closed) accounts, and a checked_from_bytes_with_discriminator::<T> per account. Only the type and a label differ between copies.

Proposed helper

Add a sibling to the existing family:

pub async fn try_fetch_program_zero_copy_accounts<T: Pod + PrecomputedDiscriminator>(
    &self,
    program_id: &Pubkey,
    commitment: CommitmentConfig,
) -> Result<Vec<(Pubkey, ZeroCopyAccountOwnedData<T>)>>

• Builds the RpcProgramAccountsConfig with a single Memcmp filter at offset 0 carrying T's discriminator, base64 encoding, and the supplied commitment.
• Skips accounts with lamports == 0.
• Deserializes each remaining account via ZeroCopyAccountOwnedData<T>, the same checked_from_bytes_with_discriminator path the other helpers use.
• Returns each account paired with its pubkey.

Take commitment as a parameter to match try_fetch_zero_copy_data_with_commitment, and optionally add a default-commitment convenience wrapper mirroring try_fetch_zero_copy_data.

Motivation

Several callers reimplement this scan-and-deserialize loop. A shared helper removes the duplication and keeps the discriminator-filter and skip-empty details in one place, consistent with the rest of the zero-copy fetch family.