[upstream #17201] Strip assistant prefill for Claude 4.6+ on Bedrock in the agents client #398

@librechat/agents strips a trailing assistant prefill only in CustomAnthropic (direct Anthropic and Vertex). LibreChat-AI/agents#621 widened that gate to Claude 4.6 and every later release, but CustomChatBedrockConverse (src/llm/bedrock/index.ts) never calls it: the stream, the local non-streaming converter and the inherited non-streaming path all send the converted messages as is. A Bedrock request for Claude 4.6+ or any 5.x model (anthropic.claude-sonnet-4-6, us.anthropic.claude-opus-5-5-v1:0, global.anthropic.claude-haiku-5-5, and application inference profiles over them) whose history ends with an assistant turn is rejected with a 400. Reproduces on agents main (v4.0.6) independent of LibreChat-AI#17201.

Fix in the agents SDK: apply the same modelDisallowsAssistantPrefill gate on the Bedrock Converse path, re-anchoring a tail cache point that rode the stripped message, and leave non-Anthropic Bedrock models untouched. Then bump @librechat/agents in LibreChat.