Fix microagent loading with trailing slashes and nested directories

[xingyaoww]

Fixes #6237

This PR fixes the issue with microagent loading when directory paths have trailing slashes. The solution:

1. Refactors get_microagents_from_selected_repo to use the more robust load_microagents_from_dir function
2. Adds test cases for:
   • Loading microagents from directories with trailing slashes
   • Loading microagents from nested directories

The fix is more general than just handling trailing slashes, as it uses the existing load_microagents_from_dir function which already handles various path formats correctly using Path objects and rglob.

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:57578b9-nikolaik   --name openhands-app-57578b9   docker.all-hands.dev/all-hands-ai/openhands:57578b9