[gemini] Handle incomplete converstations (#21008)

Conversations are limited in size. This can lead to invalid structures
being rejected by the Gemini API.
This is now handled to ensure that API requests are valid.

Signed-off-by: Holger Friedrich <mail@holger-friedrich.de>

bundles/org.openhab.binding.gemini/src/main/java/org/openhab/binding/gemini/internal/api/GeminiApiClient.java

@@ -162,7 +162,8 @@ public class GeminiApiClient {
                 case TOOL_RETURN: {
                     String name = pendingToolCallNames.poll();
                     if (name == null) {
-                        name = "";
+                        logger.trace("skipping orphaned TOOL_RETURN");
+                        break; // TOOL_RETURN without preceding TOOL_CALL - ignore
                     }
                     GeminiFunctionResponse fr = new GeminiFunctionResponse(name, Map.of("result", msg.content()));
                     GeminiPart part = new GeminiPart(null, null, fr, null);
@@ -174,6 +175,18 @@ public class GeminiApiClient {
             }
         }
 
+        while (!contents.isEmpty()) {
+            GeminiContent first = contents.getFirst();
+            List<GeminiPart> firstParts = first.parts();
+            boolean isValidFirst = ROLE_USER.equals(first.role()) && firstParts != null
+                    && firstParts.stream().anyMatch(p -> p.text() != null);
+            if (isValidFirst) {
+                break;
+            }
+            logger.trace("removing leading invalid content entry with role '{}'", first.role());
+            contents.removeFirst();
+        }
+
         List<GeminiTool> geminiTools = null;
         if (!tools.isEmpty()) {
             List<GeminiFunctionDeclaration> functions = new ArrayList<>();