[Fix]: make sure to track opened PRs using Git MCP

[malhotra5]

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

• PRs opened by agent is properly tracked in conversation metadata

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

There are two fixes here

1. User ID is None on OSS, which prevents tracking opened PRs in convo metadata; we've made user_id optional now
2. The header attribute was mismatched between the mcp client and server for the conversation id, which is also fixed now

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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:bb440f3-nikolaik   --name openhands-app-bb440f3   docker.all-hands.dev/all-hands-ai/openhands:bb440f3