Validate inputs in create_request

[iexwr]

Context

create_request in src/request.rs stores amount, memo, and expires_in without validating their values. A caller can create a request with amount == 0, expires_in == 0, or an arbitrarily long memo.

Acceptance criteria

• create_request aborts with a descriptive panic when amount <= 0.
• create_request aborts when expires_in == 0.
• memo longer than 128 bytes is rejected.
• Each case has a unit test in the #[cfg(test)] module of src/request.rs.

Technical notes

• Check amount with amount <= 0 before writing storage.
• Cap memo with memo.len() (byte length for String).
• Soroban panics become transaction failures, so keep messages specific.

Testing

• Add create_request_rejects_zero_amount, create_request_rejects_zero_expiry, create_request_rejects_long_memo.
• make test must pass and make lint must stay clean.