Add conversation start and stop endpoints

[tofarr]

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End-user friendly description of the problem this fixes or functionality this introduces.

Added new API endpoints that allow users to explicitly start and stop conversations. This gives users more control over conversation lifecycle management, enabling them to manually decide which conversations are running.

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Summarize what the PR does, explaining any non-trivial design decisions.

This PR adds two new REST API endpoints to manage conversation lifecycle:

1. /api/conversations/{conversation_id}/start - Starts a previously stopped conversation
2. /api/conversations/{conversation_id}/stop - Stops a running conversation without deleting it

The implementation leverages the existing conversation manager to handle the actual starting and stopping of agent loops. The endpoints include proper error handling and status reporting, returning appropriate HTTP status codes and messages.

Additionally, this PR includes fixes for nested runtimes to handle restarts more effectively and fixes for nested mount volumes, ensuring that conversations can be reliably started and stopped in containerized environments.

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Link of any specific issues this addresses:

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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:40fc56e-nikolaik   --name openhands-app-40fc56e   docker.all-hands.dev/all-hands-ai/openhands:40fc56e

[openhands-ai]

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@OpenHands please fix the failing actions on PR #8883

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