LSP Server computeDiagnostics overwrites ERRORS with HINTS

### Apache NetBeans version

Apache NetBeans 30

### What happened

# Bug Report: LSP Server `computeDiagnostics` overwrites ERRORS with HINTS

### Component
Java Language Server (`java.lsp.server`)

### File
`java.lsp.server/src/org/netbeans/modules/java/lsp/server/protocol/TextDocumentServiceImpl.java`

### Description
There is a logic flaw in the `computeDiagnostics` method inside `TextDocumentServiceImpl.java` that causes valid Java compilation errors to randomly disappear from the client or be completely overwritten by minor code hints.

When the LSP client requests both `ERRORS` and `HINTS` simultaneously (or when they are computed in the same batch), the method retrieves the errors successfully but fails to properly merge them with the hints. Instead of aggregating both into a single `List`, the `result` list was being overwritten.

### The Bug
In the original implementation of `computeDiagnostics(String uri, EnumSet types)` (around line 2150):
```java
List result = Collections.emptyList();
if (types.contains(ErrorProvider.Kind.ERRORS)) {
result = computeDiags(uri, -1, ErrorProvider.Kind.ERRORS, originalVersion, docHolder);
}
if (types.contains(ErrorProvider.Kind.HINTS)) {
// BUG: This completely overwrites the ERRORS computed above!
result = computeDiags(uri, -1, ErrorProvider.Kind.HINTS, originalVersion, docHolder);
}
r.complete(result);
```
Because `result` is assigned directly without appending, if both `ERRORS` and `HINTS` are requested, the `HINTS` computation completely eradicates the `ERRORS` from the final payload sent back to the LSP client.

### Proposed Fix
The `result` collection must be instantiated as an `ArrayList` and both types of diagnostics must be merged into it using `.addAll()`.

```java
List result = new ArrayList<>();
if (types.contains(ErrorProvider.Kind.ERRORS)) {
result.addAll(computeDiags(uri, -1, ErrorProvider.Kind.ERRORS, originalVersion, docHolder));
}
if (types.contains(ErrorProvider.Kind.HINTS)) {
result.addAll(computeDiags(uri, -1, ErrorProvider.Kind.HINTS, originalVersion, docHolder));
}
r.complete(result);
```

### Impact
Without this fix, any client-side tool or background scanner that requests full diagnostics for a file via the `nbls.get.diagnostics` command will receive an incomplete picture of the file's health, leading to valid compilation errors being hidden from the developer.

### Language / Project Type / NetBeans Component

_No response_

### How to reproduce

When the LSP client requests both ERRORS and HINTS simultaneously (or when they are computed in the same batch), the method retrieves the errors successfully but fails to properly merge them with the hints. Instead of aggregating both into a single List, the result list was being overwritten.

### Did this work correctly in an earlier version?

No / Don't know

### Operating System

Windows

### JDK

26

### Apache NetBeans packaging

Apache NetBeans binary zip

### Anything else

this is related to this Pull Request for Netbeans-vscode:
https://github.com/apache/netbeans-vscode/pull/33

### Are you willing to submit a pull request?

No

Open java.lsp.server/src/org/netbeans/modules/java/lsp/server/protocol/TextDocumentServiceImpl.java and inspect computeDiagnostics around line 2150, focusing on how ERRORS and HINTS are returned together. Verify that a request containing both kinds preserves both diagnostic sets in the completed result, and check the relevant LSP diagnostics tests if available.

Written by the indexing model from the issue text.