chore: pass bedrock_client with UNSIGNED to prevent IMDS timeout from langchain_aws

[cosmin-staicu]

• PR chore: use botocore.UNSIGNED to prevent IMDS timeout in bedrock #692 fixed the bedrock-runtime client IMDS timeout, but langchain_aws's ChatBedrockConverse.validate_environment() creates its own bedrock control plane client when
  bedrock_client is not passed, triggering the same credential chain walk and ~2s IMDS timeout
• Added get_bedrock_client() using botocore.UNSIGNED and pass it as bedrock_client kwarg in both UiPathChatBedrockConverse and UiPathChatBedrock constructors
• Moved total_max_attempts: 1 and read_timeout=300 to only apply to the bedrock-runtime client; the bedrock control plane client uses default retry/timeout behavior
• Extracted _build_session() and _unsigned_config() helpers to share session and config logic between both clients