Challenge: Model Anchor Program Structure in Rust

Anchor programs follow a specific pattern: instructions contain account metadata and serialized data. You'll model this structure using Rust structs and enums.

Your Task

Implement build_instruction(program_id, user_key, instruction_name) that constructs an Anchor-style instruction:

1. Encode the instruction name as bytes and compute its length
2. Build account metadata for two accounts:
   • User account: signer=true, writable=true
   • System program ("11111111111111111111111111111111"): signer=false, writable=false
3. Return a formatted string: "program:<id>,accounts:<count>,data_len:<len>"

This mirrors Anchor's #[derive(Accounts)] pattern where each instruction declares which accounts it needs and how they should be validated.

Rust Concepts Used:

• Structs with boolean fields
• Vec collection
• String formatting with format!()
• Byte conversion with .as_bytes()