RestRequest parameter parsing should not cause 500 error

Currently, RestRequest throws IllegalStateExceptions in a few places which leads
to 500 errors on the client side and e.g. turn up as warnings in the
"rest.suppressed" error log. By changing them to IllegalArgumentExceptions we
can change these to 400 (Bad Request) error codes.

Closes elastic#112604

server/src/main/java/org/elasticsearch/rest/RestRequest.java

@@ -297,7 +297,7 @@ public final BytesReference requiredContent() {
         if (hasContent() == false) {
             throw new ElasticsearchParseException("request body is required");
         } else if (xContentType.get() == null) {
-            throw new IllegalStateException("unknown content type");
+            throw new IllegalArgumentException("unknown content type");
         }
         return content();
     }
@@ -561,12 +561,12 @@ public final Tuple<XContentType, BytesReference> contentOrSourceParam() {
         String source = param("source");
         String typeParam = param("source_content_type");
         if (source == null || typeParam == null) {
-            throw new IllegalStateException("source and source_content_type parameters are required");
+            throw new IllegalArgumentException("source and source_content_type parameters are required");
         }
         BytesArray bytes = new BytesArray(source);
         final XContentType xContentType = parseContentType(Collections.singletonList(typeParam));
         if (xContentType == null) {
-            throw new IllegalStateException("Unknown value for source_content_type [" + typeParam + "]");
+            throw new IllegalArgumentException("Unknown value for source_content_type [" + typeParam + "]");
         }
         return new Tuple<>(xContentType, bytes);
     }

server/src/test/java/org/elasticsearch/rest/RestRequestTests.java

@@ -129,7 +129,7 @@ public void testContentOrSourceParam() throws IOException {
                 .contentOrSourceParam()
                 .v2()
         );
-        e = expectThrows(IllegalStateException.class, () -> contentRestRequest("", Map.of("source", "stuff2")).contentOrSourceParam());
+        e = expectThrows(IllegalArgumentException.class, () -> contentRestRequest("", Map.of("source", "stuff2")).contentOrSourceParam());
         assertEquals("source and source_content_type parameters are required", e.getMessage());
     }
 
@@ -245,7 +245,7 @@ public void testRequiredContent() {
                 .requiredContent()
         );
         assertEquals("request body is required", e.getMessage());
-        e = expectThrows(IllegalStateException.class, () -> contentRestRequest("test", null, Collections.emptyMap()).requiredContent());
+        e = expectThrows(IllegalArgumentException.class, () -> contentRestRequest("test", null, Collections.emptyMap()).requiredContent());
         assertEquals("unknown content type", e.getMessage());
     }