fix(specs): make shared MAX_SIGNERS checks type-safe

[Stevengre]

Summary

This PR fixes the shared multisig initialize specs so they keep using MAX_SIGNERS directly while type-checking in both inclusion contexts.

The root issue was that the shared spec is included by both spl-token and p-token, but MAX_SIGNERS does not have the same type in both paths. Replacing the old magic numbers with MAX_SIGNERS therefore introduced a type mismatch in the proof/runtime-verification path.

Testing

• cargo +nightly-2025-02-16 clippy --manifest-path p-token/Cargo.toml -Zunstable-options --all-targets --all-features -- --deny=warnings --deny=clippy::arithmetic_side_effects
• cargo +nightly-2025-02-16 check -p spl-token --features runtime-verification
• GitHub Actions: Format & Lint p-token passed in run 22931335543