fix(ffi): scope jid argument ownership

[Andiveli]

Closes #374

Summary

• keep Rust ownership of JID C strings across borrowed FFI calls
• decode empty FFI contact results without constructing a slice from a null pointer
• preserve the existing ABI

Changes

File	Change
whatsrust/src/models.rs	Add scoped ownership for C-compatible JIDs
whatsrust/src/queries.rs	Use scoped arguments and guard empty results
whatsrust/src/message_send.rs	Keep JID owners alive through outbound FFI calls

Test plan

• cargo test --manifest-path whatsrust/Cargo.toml — 27 passed
• Formatting and diff checks passed

Checklist

• Linked approved issue
• Conventional commit
• No generated or unrelated changes