refactor: remove pairing_file field and state param from RemotePairingClient

[maxslarsson]

Motivation

This fixes a borrow conflict where you can't save the pairing file while the RemotePairingClient is still alive, since the struct held a &mut borrow for its entire lifetime:

error[E0502]: cannot borrow `rpf` as immutable because it is also borrowed as mutable
  --> src/rsd.rs:103:9
   |
94 |     let mut rpc = RemotePairingClient::new(conn, SENDING_HOST, &mut rpf);
   |                                                                -------- mutable borrow occurs here
...
103|         rpf.write_to_file(&path)
   |         ^^^ immutable borrow occurs here
...
109|     rpc.create_tcp_listener();
   |     --- mutable borrow later used here

By moving the &mut RpPairingFile to a parameter on connect(), the borrow is released when connect() returns, so callers can freely use both rpc and rpf afterwards.

Changes Made

• Remove pairing_file: &'a mut RpPairingFile from the RemotePairingClient struct, eliminating the 'a lifetime parameter. Instead, connect() accepts &mut RpPairingFile by argument. The mutable borrow is now scoped to the function call rather than the struct's lifetime.
• Remove the state: S generic from connect, pair, and request_pair_consent. Callers can capture any needed state via move closures instead.
• Update all call sites in tools/ and ffi/ to match the new signatures.

TypeState Pattern

This struct would be a good candidate for the TypeState pattern since it transitions through two distinct phases (unconnected, paired) where different operations are valid at each phase. Encoding these states as type parameters would make it a compile-time error to call create_tcp_listener() or encryption_key() before pairing completes, rather than relying on runtime checks and placeholder cipher values. I wasn't sure if that large of an API refactor would be in the interest of the project though. It would also almost certainly require validate_pairing/pair to become internal functions, since if they consume self to return a new type state, then connect can't do the "try validate first, fall back to pair on error" flow.