Varifier doesn't always trigger

Varifier doesn't always seem to correctly trigger.

It does work for:
```java
Optional test = Optional.of(1);
```
But not for:
```java
var optInt = Optional.of(1);
Optional test = optInt;
```
And also not for:
```java
private static Optional getOptInt() {
return Optional.of(1);
}
...
Optional test = getOptInt();
```

Using the following versions:
Java: 25.0.2
Gradle: 9.4.1
Errorprone: 2.50.0
Errorprone gradle plugin: 5.1.0

Start by reproducing the Varifier behavior with the three Java snippets in the issue under Java 25.0.2, Error Prone 2.50.0, and the Error Prone Gradle plugin 5.1.0. Compare which snippets produce diagnostics, then locate the Varifier implementation and add regression coverage showing the expected trigger behavior for each form.

Written by the indexing model from the issue text.