Add deployment field to agent dispatch in TokenSource

[xianshijing-lk]

Summary

• Add AgentDeployment property to TokenSourceFetchOptions for specifying target deployment
• Add Deployment field to AgentDispatch class with JSON serialization
• Update BuildRequest() to include deployment when any agent field is set

Leave deployment empty to target the production deployment.

Related PRs

• node-sdks: Add deployment to CreateDispatchOptions node-sdks#675
• client-sdk-js: Add deployment to TokenSourceFetch options client-sdk-js#1971
• client-sdk-swift: feat: add deployment field to agent dispatch client-sdk-swift#1043
• client-sdk-flutter: feat: add deployment field to agent dispatch client-sdk-flutter#1111
• client-sdk-android: feat: add deployment field to agent dispatch client-sdk-android#968

Verification Steps

1. Create a Unity project with the LiveKit SDK
2. Use TokenSourceEndpoint or TokenSourceSandbox with TokenSourceFetchOptions:

   var options = new TokenSourceFetchOptions
   {
       RoomName = "test-room",
       ParticipantIdentity = "user-1",
       AgentName = "my-agent",
       AgentDeployment = "staging"  // New field
   };
   var details = await tokenSource.FetchConnectionDetails(options);

3. Verify the JSON request includes "deployment": "staging" in the agent dispatch
4. Test with empty deployment (production default)

🤖 Generated with Claude Code

[MaxHeimbrock]

Right now my status is:
When I put anything in that field, the agent is not deployed. I tried production, staging, abcd. Whatever I do, I don't get an error but the agent is not deployed. If I leave the field empty, it is deployed. What is the expected behaviour of the field to test against?