Fix race condition in ShutdownListener constructor

[matt-richardson]

Summary

Fixes #2927

This PR fixes a race condition in the ShutdownListener constructor that causes a NullReferenceException when the shutdown token fires during initialization.

The Problem

The constructor was registering the cancellation callback before assigning _innerListener:

_shutdownToken = shutdownToken;
_shutdownRegistration = shutdownToken.Register(Cancel);  // Registers Cancel callback
_innerListener = innerListener;  // Assigned AFTER

If the shutdown token fired in the microseconds between registering the callback and assigning _innerListener, the Cancel() method would be invoked with _innerListener still null, resulting in a NullReferenceException at line 40.

The Fix

This PR swaps the order to assign _innerListener before registering the callback:

_shutdownToken = shutdownToken;
_innerListener = innerListener;  // Now assigned FIRST
_shutdownRegistration = shutdownToken.Register(Cancel);  // Safe to register

This ensures _innerListener is always initialized when Cancel() is invoked, eliminating the race condition.

Testing

• ✅ Build verified successfully
• This is a defensive fix for a rare production race condition

Impact

This is a critical fix for production stability. The race condition is rare but when it occurs, it causes the entire WebJob to crash during startup.

[matt-richardson]

@jviau - could I trouble you to take a look at this PR as well?
(sorry to push my luck here)

[jviau]

/azp run webjobs-sdk.public

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[matt-richardson]

Thanks @jviau! Appreciate it.