Java: union of arrays produces a compilation failure

### Describe the bug

The following:

```ts
export type X = A[] | B[] | C;
```

Fails to compile in Java, because type erasure turns this into 2 arrays.

The following does work:

```ts
export type X = (A | B)[] | C;
```

jsii-compiler should reject the former.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

If it passes the compiler it shouldn't fail packaging.

### Current Behavior

The opposite.

### Reproduction Steps

SEe description.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### SDK version used

-

### Environment details (OS name and version, etc.)

-

Start by tracing the jsii-compiler validation path for exported union types and reproduce the two TypeScript declarations through Java packaging. Compare the accepted union-of-arrays case with the working parenthesized form, then add coverage showing that the former is rejected before packaging. Done means the compiler reports the invalid Java-erasure case clearly and the valid form still packages.

Written by the indexing model from the issue text.