Adapt verl-omni's diffusion agent loop and ray trainer to verl-project/verl#6129, which removed AsyncLLMServerManager and made AgentLoopManager / AgentLoopWorker consume an LLMServerClient produced by a separately-owned LLMServerManager.

verl-omni changes:

• DiffusionAgentLoopWorker.init now takes (config, llm_client, teacher_client, reward_loop_worker_handles), matching the positional contract that AgentLoopManager.create() uses when spawning workers. _get_rollout_and_model_config was also dropped upstream, so the config slicing is inlined to keep the diff minimal.
• ray_diffusion_trainer now creates an LLMServerManager first, hands its client to AgentLoopManager.create(), and uses llm_server_manager.get_replicas() (instead of async_rollout_manager.rollout_replicas) to wire the CheckpointEngineManager. This mirrors the new pattern in upstream verl/trainer/ppo/ray_trainer.py.
• tests/agent_loop/test_diffusion_agent_loop.py is updated for the new API; in standalone test mode LLMServerManager spins up its own replicas via rollout.nnodes / n_gpus_per_node.

Pin / docs / CI:

• Bump the pinned verl commit to a4351480 (the merge commit of #5209), which is the first commit that ships verl/experimental/reward_loop/router/ in the wheel AND contains the #6129 refactor that this change adapts to. With this commit, the workaround in PR [doc, ci] chore: bump verl pin past router packaging fix (#5209) #51 (clone + editable install) is no longer required.
• Restore the simple uv pip install git+...@<commit> install line in docs/start/install.md.
• Bump the same pin in .github/workflows/{cpu_unit_tests,sanity,type-coverage-check}.yml.

This is a BREAKING change because DiffusionAgentLoopWorker.init signature changed. Any downstream code that subclasses or directly instantiates DiffusionAgentLoopWorker must switch from (servers, load_balancer_handle, teacher_servers, teacher_load_balancer_handle) to (llm_client, teacher_client). No public CLI/config surface is affected.

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