Fix: prevent empty embedding batches from causing memory spikes (#6230)

Root cause: When splitting failed embedding batches, mid=0 for single
items created empty chunks. These empty chunks were then processed,
creating hundreds of no-op tasks that consumed memory.

Crash pattern from logs:
- 600+ 'batch_size=0' embedding tasks created
- Memory spiked 531 MB → 4.9 GB
- Pod crashed

Fixes:
1. Skip empty chunks before creating tasks
2. Guard chunk splits to prevent empty slices (mid = max(1, len//2))
3. Break early if all chunks are empty

This prevents the asyncio.gather() from creating thousands of empty
coroutines that exhaust memory.

letta/llm_api/openai_client.py

@@ -779,6 +779,10 @@ class OpenAIClient(LLMClientBase):
             task_metadata = []
 
             for start_idx, chunk_inputs, current_batch_size in chunks_to_process:
+                if not chunk_inputs:
+                    logger.warning(f"Skipping empty chunk at start_idx={start_idx}")
+                    continue
+
                 logger.info(
                     f"Creating embedding task: start_idx={start_idx}, batch_size={len(chunk_inputs)}, "
                     f"first_input_len={len(chunk_inputs[0]) if chunk_inputs else 0}, "
@@ -788,6 +792,10 @@ class OpenAIClient(LLMClientBase):
                 tasks.append(task)
                 task_metadata.append((start_idx, chunk_inputs, current_batch_size))
 
+            if not tasks:
+                logger.warning("All chunks were empty, skipping embedding request")
+                break
+
             task_results = await asyncio.gather(*tasks, return_exceptions=True)
 
             failed_chunks = []
@@ -801,17 +809,21 @@ class OpenAIClient(LLMClientBase):
                             f"Embeddings request failed for batch starting at {start_idx} with size {current_size}. "
                             f"Reducing batch size from {current_batch_size} to {new_batch_size} and retrying."
                         )
-                        mid = len(chunk_inputs) // 2
-                        failed_chunks.append((start_idx, chunk_inputs[:mid], new_batch_size))
-                        failed_chunks.append((start_idx + mid, chunk_inputs[mid:], new_batch_size))
+                        mid = max(1, len(chunk_inputs) // 2)
+                        if chunk_inputs[:mid]:
+                            failed_chunks.append((start_idx, chunk_inputs[:mid], new_batch_size))
+                        if chunk_inputs[mid:]:
+                            failed_chunks.append((start_idx + mid, chunk_inputs[mid:], new_batch_size))
                     elif current_size > min_chunk_size:
                         logger.warning(
                             f"Embeddings request failed for single item at {start_idx} with size {current_size}. "
                             f"Splitting individual text content and retrying."
                         )
-                        mid = len(chunk_inputs) // 2
-                        failed_chunks.append((start_idx, chunk_inputs[:mid], 1))
-                        failed_chunks.append((start_idx + mid, chunk_inputs[mid:], 1))
+                        mid = max(1, len(chunk_inputs) // 2)
+                        if chunk_inputs[:mid]:
+                            failed_chunks.append((start_idx, chunk_inputs[:mid], 1))
+                        if chunk_inputs[mid:]:
+                            failed_chunks.append((start_idx + mid, chunk_inputs[mid:], 1))
                     else:
                         chunk_preview = str(chunk_inputs)[:500] if chunk_inputs else "None"
                         logger.error(