Extract test Argon2 parameters into reusable helper function

[Copilot]

Addresses review feedback on #239 to reduce duplication of test Argon2 parameters across test files.

Changes

• Added test_argon2_params() helper in test_utils.rs returning weak params (128 KiB, 1 iteration, 1 parallelism)
• Replaced 11 inline parameter definitions (9 in auth.rs, 2 in lib.rs) with calls to the helper

Usage

// Before
let password_hash = password::hash_string_with_params(
    "testpassword",
    Some(password::Argon2Params {
        memory_kib: 128,
        iterations: 1,
        parallelism: 1,
    }),
).unwrap();

// After
let password_hash = password::hash_string_with_params(
    "testpassword",
    Some(test_argon2_params()),
).unwrap();

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