Fix - Make Remote runtime use /start for both new and resume

[raymyers]

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• [ ] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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

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

Simplify remote runtime by calling the /start endpoint on both new runtimes and resumes, since runtime-api can handle both cases this was duplicating the server-side logic.

We've recently seen trouble resuming Remote runtimes in OpenHands Cloud, which may be related to multiple being created in a race condition, so the hope is to improve consistency by letting runtime-api decide which existing runtime to use if any.

An error associated is

Error connecting to conversation <ID>: Could not find existing runtime for SID: <ID>"

Logged from here: https://github.com/All-Hands-AI/OpenHands/blob/2693360ad026e6e0f04b4de6d23b026e3cfc0d55/openhands/runtime/impl/remote/remote_runtime.py#L121

This will ignore the attach_to_existing option on create (not on close). That was added in #4329 (Oct 2024) and I don't think it currently adds anything useful on RemoteRuntime create, but @tofarr and @rbren may have more context.

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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:364c571-nikolaik   --name openhands-app-364c571   docker.all-hands.dev/all-hands-ai/openhands:364c571

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[raymyers]

We have merge conflicts now and I think it's best to redo this after Nested Runtimes is in.