Gateway responder's StartHandshake doesn't return a packet - it just transitions
to KKTExchange state and waits for client's KKT request. Fixed handle_client_hello()
to not expect a response packet.
## Changes
### Add BOOTSTRAP_SESSION_ID constant
- `common/nym-lp/src/packet.rs`: Add `pub const BOOTSTRAP_SESSION_ID: u32 = 0`
- Document that session_id=0 is only used for ClientHello bootstrap
- Export from lib.rs for public use
### Fix handle_client_hello() logic
- `gateway/src/node/lp_listener/handler.rs:201-225`: - Remove expectation of SendPacket action from StartHandshake
- Responder transitions to KKTExchange without sending
- Store state machine and close connection
- Client sends KKT request on next connection with computed session_id
### Use constant throughout codebase
- `gateway/src/node/lp_listener/handler.rs:115`: Use BOOTSTRAP_SESSION_ID in routing
- `nym-registration-client/src/lp_client/client.rs:259`: Use constant instead of literal 0
## Protocol Flow (Fixed)
```
Connection 1: Client sends ClientHello (session_id=0)
→ Gateway stores state, closes (no response)
Connection 2: Client sends KKT request (session_id=X)
→ Gateway finds state, processes, responds
Connection 3+: Handshake continues until complete...
```
## Testing
- All 13 unit tests pass
- Real test: docker/localnet + nym-gateway-probe (next step)
Fixes: nym-v9un
Unblocks: nym-21th
durch
e8a3d5720c