Remove unnecessary verify_string_with_params wrapper

[Copilot]

Addresses review feedback on #239 to remove a pointless wrapper function.

verify_string_with_params was unnecessary because Argon2 verification always extracts parameters from the hash itself—the params argument was ignored.

Changes

• Removed verify_string_with_params function
• Moved implementation directly into verify_string

// Before: wrapper that ignored its params argument
pub fn verify_string_with_params(input: &str, hash: &str, _params: Option<Argon2Params>) -> Result<bool, Error> { ... }
pub fn verify_string(input: &str, hash: &str) -> Result<bool, Error> {
    verify_string_with_params(input, hash, None)
}

// After: single function with the actual implementation
pub fn verify_string(input: &str, hash: &str) -> Result<bool, Error> { ... }

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