Enhance Bedrock Agents support to accommodate Messages, ToolNode

[3coins]

The original PR that introduced Bedrock Agents support had a few shortcomings w.r.t. use of deprecated components and no message support which is central to the way LangChain components work.

1. Even though there is no LLM present here (agent manages the LLM calls), a lot of the LangChain components work around messages which is central to it's working. We should follow up with a PR to support messages.
2. The LangGraph integration test is using the ToolExecutor, which has been deprecated in favor of ToolNode, which works only with messages. To future proof this work, we should move to use ToolNode in LangGraph samples and tests.

Originally posted by @3coins in #152 (comment)