## Description

This PR fixes `PrefixCacheAffinityRouter` to use deployment-specific
prefix tree actors instead of a single shared global actor. This
resolves replica ID conflicts that occur when multiple deployments use
the router (e.g., in prefill-decode disaggregation with data
parallelism).

### Problem

Previously, all `PrefixCacheAffinityRouter` instances shared a single
detached actor named `LlmPrefixTreeActor`. In multi-deployment scenarios
like PD disaggregation with DP, this caused:

```
KeyError: Replica(id='bzw6m3yr', deployment='Decode:deepseek', app='deepseek-pd-nccl')
```

This happened because Prefill and Decode deployments (each with 16 DP
replicas) were all tracked in the same prefix tree, causing replica ID
collisions when the router tried to route requests.

### Solution

Modified `PrefixCacheAffinityRouter.initialize_state()` to create
deployment-specific prefix tree actors using **namespaces** derived from
`SERVE_NAMESPACE`, app name, and deployment name:

- Single deployment: `serve::LlmPrefixTreeActor`
- PD scenario:
`serve::deepseek-pd-nccl::Prefill:deepseek::LlmPrefixTreeActor` and
`serve::deepseek-pd-nccl::Decode:deepseek::LlmPrefixTreeActor`

Each deployment now maintains its own isolated prefix tree state,
preventing replica ID conflicts.

## Changes

-
`python/ray/llm/_internal/serve/routing_policies/prefix_aware/prefix_aware_router.py`
  - Imports `SERVE_NAMESPACE` from `ray.serve._private.constants`
- Builds a namespace from `SERVE_NAMESPACE`, `app_name`, and
`deployment_name` (e.g., `serve::app::deployment`)
  - Creates the actor with this deployment-specific namespace

## Testing

- Validated manually with PD + DP deployments using DeepSeek-V2-Lite

## Impact

- Enables `PrefixCacheAffinityRouter` to work correctly with PD
disaggregation + DP
- No breaking changes for single deployment scenarios (backward
compatible)
- Users can now use prefix-aware routing in complex multi-deployment
scenarios

---------

Signed-off-by: Seiji Eicher <seiji@anyscale.com>
Signed-off-by: peterxcli <peterxcli@gmail.com>