[identity] use identitySearchSocket instead of socket.current

Summary:
This addresses https://linear.app/comm/issue/ENG-9971/[error-invalid-state-err]-in-identitysearchprovider

We should be using identitySearchSocket instead of socket.current could be null before we properly set it

Test Plan:
cargo check

Didn't get to testing this properly. Putting up this diff for now to get it into review, but I'll ensure we're not getting spammed with:
```
ERROR  [Error: INVALID_STATE_ERR]
```
before landing

Reviewers: kamil, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14094

lib/identity-search/identity-search-context.js

@@ -203,7 +203,7 @@ function IdentitySearchProvider(props: Props): React.Node {
         const heartbeat: Heartbeat = {
           type: identitySearchMessageToServerTypes.HEARTBEAT,
         };
-        socket.current?.send(JSON.stringify(heartbeat));
+        identitySearchSocket.send(JSON.stringify(heartbeat));
       }
     };