fix: null socket on bb process exit/error in native_socket.ts

[AztecBot]

Summary

Defensive hardening of the native socket backend used by ChonkBatchVerifier.

When the bb helper process crashes (e.g. SIGABRT), the socket was destroyed but the reference was not nulled. This meant subsequent calls to call() would pass the if (!this.socket) guard and attempt to write to a destroyed socket. While the existing socket error/end handlers would eventually reject callbacks via async error events, nulling the socket makes the guard clause reject immediately — fail-fast rather than relying on async error propagation.

Changes:

• Null this.socket after destroy() in process error handler
• Null this.socket after destroy() in process exit handler
• Null this.socket after destroy() in cleanup() method

Test plan

• Existing unit tests should pass — this is a defensive null assignment, no behavior change for healthy paths
• On process crash, calls to call() now throw Socket not connected immediately instead of writing to a destroyed socket and waiting for the async error event

ClaudeBox log: https://claudebox.work/s/1164aa0ab601318b?run=5