fix(headless): actually check readiness in pregenerateChunks

#5024: PrepareWorld.step() loops until worldRenderer.pregenerateChunks()
returns true or 5 seconds elapse. HeadlessWorldRenderer.pregenerateChunks()
was an unimplemented stub that always returned false, so headless runs
(including every MTE test) always burned the full 5 second wait
regardless of how quickly the world actually generated.

## Fix

Implemented pregenerateChunks() using the same chunk-loaded/local-view
check updateChunksInProximity() already performs for per-frame updates:
advance chunkProvider, force a full rescan of the current view region,
and report done once every chunk in it is loaded.

The rescan has to be forced: updateChunksInProximity's own
"did the camera move" shortcut is keyed off chunkPos, which never
changes in headless (NullCamera reports a constant position), so an
unforced call would never re-examine chunk state and permanently report
"nothing pending" from its zero-value default - a different way to end
up with the always-true failure mode keturn already ran into on a bare
`return true`, without ever confirming anything actually loaded.

## Verification

:engine:compileJava clean. Ran the full integrationenvironment test
suite (engine-tests:integrationTest, 18 MTE test classes covering
per-class and per-method lifecycles back-to-back in the same JVM,
34 tests total) - the exact scenario keturn's earlier attempt broke
("first test fine, next one had problems") - all 34 pass, 0 failures.

Fixes #5024

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: soloturn

engine/src/main/java/org/terasology/engine/core/subsystem/headless/renderer/HeadlessWorldRenderer.java

@@ -132,8 +132,12 @@ public void dispose() {
 
     @Override
     public boolean pregenerateChunks() {
-        // TODO Auto-generated method stub
-        return false;
+        chunkProvider.update();
+        // force=true: updateChunksInProximity's "did the camera move" shortcut never fires here since the
+        // headless camera position never changes, so a forced full rescan is the only way to learn whether
+        // every chunk in view distance has actually loaded (see #5024).
+        updateChunksInProximity(true);
+        return !pendingChunks;
     }
 
     @Override