feat(spl): cheatcode for multisig

[Stevengre]

1. Unpack/Pack function recognition - Added Multisig::unpack_from_slice and Multisig::pack_into_slice to the SPL function predicates
2. Buffer size calculation - Added a #maybeDynamicSize rule for Multisig data buffers (355 bytes), matching the SPL Token Multisig::LEN constant
3. New cheatcode rule - [cheatcode-is-spl-multisig] that:
   - Intercepts calls to cheatcode_is_spl_multisig
   - Creates a symbolic SPLDataBuffer with the Multisig structure:
   • m: u8 - number of signers required
   • n: u8 - number of valid signers
   • is_initialized: bool
   • signers: [Pubkey; 11] - array of 11 signer public keys
     - Initializes RefCell borrow metadata with correct buffer size (355)
     - Adds appropriate ensures clauses constraining symbolic values

[Stevengre]

Tested by:

    cheatcode_is_spl_multisig(multisig);
    let mut ms = Multisig::unpack_unchecked(&multisig.data.borrow()).unwrap();
    ms.m = 2;  // require 2 signers
    ms.n = 3;  // 3 valid signers
    ms.is_initialized = true;
    Multisig::pack(ms, &mut multisig.data.borrow_mut()).unwrap();
    let ms2 = Multisig::unpack(&multisig.data.borrow()).unwrap();
    assert!(ms2.is_initialized);
    assert_eq!(ms2.m, 2);
    assert_eq!(ms2.n, 3);

should be added in solana-token.

[dkcumming]

LGTM

[dkcumming]

Is this needed?

[Stevengre]

No, I forgot to delete this trial.