`UnnecessaryStringBuilder` misses the split declaration/assignment form

### Error Prone version

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

### Check name

[UnnecessaryStringBuilder](https://errorprone.info/bugpattern/UnnecessaryStringBuilder)

### Description

`UnnecessaryStringBuilder` flags a `StringBuilder` local that is created and not meaningfully used (replaceable by plain string handling). The matcher keys on the combined declarator-with-initializer node `StringBuilder x = new StringBuilder(...)`. Splitting it into a separate declaration and assignment — `StringBuilder x; x = new StringBuilder(...);` — is a semantics-preserving refactor (the variable is still initialized once before any use), yet the rule stops firing entirely.

### Reproducer

```java
// BEFORE — 2 findings (on the two StringBuilder locals)
class S {
public void f() {
StringBuffer foo = new java.lang.StringBuffer('x');
StringBuilder foo2 = new StringBuilder('x'); // flagged
StringBuffer foo3 = new StringBuffer("x");
StringBuilder foo4 = new StringBuilder("x"); // flagged
}
}
```

```java
// AFTER — equivalent rewrite, 0 findings
class S {
public void f() {
StringBuffer foo;
foo = new java.lang.StringBuffer('x');
StringBuilder foo2;
foo2 = new StringBuilder('x');
StringBuffer foo3;
foo3 = new StringBuffer("x");
StringBuilder foo4;
foo4 = new StringBuilder("x");
}
}
```

### Expected behavior

`UnnecessaryStringBuilder` should report the same findings on BEFORE and AFTER, since `T x = init;` and `T x; x = init;` are observationally equivalent.

### Actual behavior

- BEFORE: 2 findings (`Prefer string concatenation over explicitly using StringBuilder#append ...`).
- AFTER: 0 findings.

The matcher only recognizes the declarator-with-initializer form and does not follow the variable when the initializer is a separate assignment statement.