Extract magic number for user account count to constant

[Copilot]

Addresses code review feedback to eliminate magic number 3 representing user account count. The coding guidelines require all magic numbers be extracted to constants, similar to how MAX_PDP_SP_COUNT is defined.

Changes

• Added USER_ACCOUNT_COUNT constant in src/constants.rs set to 3, representing the number of test user accounts (USER_1, USER_2, USER_3)

• Replaced 4 hardcoded 3 values with the constant across:

  • src/external_api/export.rs: User info export loop
  • src/commands/start/usdfc_funding/usdfc_funding_step.rs: Balance checking, token transfer, and verification loops

// Before
for i in 1..=3 {
    let name = format!("USER_{}", i);
    // ...
}

// After
for i in 1..=USER_ACCOUNT_COUNT {
    let name = format!("USER_{}", i);
    // ...
}

This centralizes the user count configuration and improves maintainability.

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