Runtime.invoke() does not populate custom_headers to invoke_bedrock_agentcore operation

custom_headers are not passed to the invoke_bedrock_agentcore operation from Runtime.invoke() method.

we need to verify the custom headers prefix and invoke it with the filtered headers, just like the cli command.
see here : https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/ba056fb5fa557cc3b80920b38befb7d6cbde6007/src/bedrock_agentcore_starter_toolkit/cli/runtime/commands.py#L958C1-L959C1

Start by locating Runtime.invoke() and compare its handling with the CLI implementation at src/bedrock_agentcore_starter_toolkit/cli/runtime/commands.py lines 958-959. Verify the custom-header prefix, filter the headers accordingly, and confirm the filtered headers are passed to invoke_bedrock_agentcore.

Written by the indexing model from the issue text.