`FormatString` false positive on a switch-expression argument to printf("%d", ...)

### Error Prone version

Error Prone 2.49.0 (javac plugin) — javac 21.0.8, source/target 21.

### Check name

`FormatString` — https://errorprone.info/bugpattern/FormatString

### Description

`FormatString` validates that `printf`-style format strings match their arguments. When the argument is an `int`-typed **switch expression** passed directly to `%d`, the check fails to resolve the argument's type and falsely reports an `illegal format conversion`. The diagnostic even names an internal sentinel class (`com.google.errorprone.bugpatterns.formatstring.FormatStringValidation$...`) as the argument type, confirming the type resolution falls back to a placeholder for switch-expression arguments.

This is a **false positive** (the strongest finding in this batch): the same value passed via a `var` temporary is accepted, but inlining the switch expression triggers a spurious error.

### Reproducer

```java
// BEFORE — 0 findings (int routed through a var temporary)
public class C {
static final int FRIDAY = 5;
public static void main(String[] args) {
int day = FRIDAY;
var numLetters = switch (day) {
case 1, 5, 7 -> 6;
case 2 -> 7;
default -> 9;
};
System.out.printf("NumLetters: %d%n", numLetters);
}
}
```

```java
// AFTER — equivalent inlining, 1 spurious finding
public class C {
static final int FRIDAY = 5;
public static void main(String[] args) {
int day = FRIDAY;
System.out.printf("NumLetters: %d%n", switch (day) {
case 1, 5, 7 -> 6;
case 2 -> 7;
default -> 9;
});
}
}
```

### Expected behavior

`FormatString` should report nothing on either form. The switch expression has static type `int`, which is compatible with `%d`, exactly like the `var` temporary in BEFORE.

### Actual behavior

- BEFORE: 0 findings.
- AFTER: 1 finding — `illegal format conversion: 'com.google.errorprone.bugpatterns.formatstring.FormatStringValidation$...' ...`

The type of a switch-expression argument is not resolved; the check substitutes an internal sentinel type and emits a false `illegal format conversion`.