Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/docs/deployment/frameworks/dify.md

@@ -0,0 +1,57 @@
+# Dify
+
+[Dify](https://github.com/langgenius/dify) is an open-source LLM app development platform. Its intuitive interface combines agentic AI workflow, RAG pipeline, agent capabilities, model management, observability features, and more, allowing you to quickly move from prototype to production.
+
+It supports vLLM as a model provider to efficiently serve large language models.
+
+This guide walks you through deploying Dify using a vLLM backend.
+
+## Prerequisites
+
+Set up the vLLM environment:
+
+```bash
+pip install vllm
+```
+
+And install [Docker](https://docs.docker.com/engine/install/) and [Docker Compose](https://docs.docker.com/compose/install/).
+
+## Deploy
+
+1. Start the vLLM server with the supported chat completion model, e.g.
+
+    ```bash
+    vllm serve Qwen/Qwen1.5-7B-Chat
+    ```
+
+1. Start the Dify server with docker compose ([details](https://github.com/langgenius/dify?tab=readme-ov-file#quick-start)):
+
+    ```bash
+    git clone https://github.com/langgenius/dify.git
+    cd dify
+    cd docker
+    cp .env.example .env
+    docker compose up -d
+    ```
+
+1. Open the browser to access `http://localhost/install`, config the basic login information and login.
+
+1. In the top-right user menu (under the profile icon), go to Settings, then click `Model Provider`, and locate the `vLLM` provider to install it.
+
+1. Fill in the model provider details as follows:
+
+    - **Model Type**: `LLM`
+    - **Model Name**: `Qwen/Qwen1.5-7B-Chat`
+    - **API Endpoint URL**: `http://{vllm_server_host}:{vllm_server_port}/v1`
+    - **Model Name for API Endpoint**: `Qwen/Qwen1.5-7B-Chat`
+    - **Completion Mode**: `Completion`
+
+    ![Dify settings screen](../../assets/deployment/dify-settings.png)
+
+1. To create a test chatbot, go to `Studio → Chatbot → Create from Blank`, then select Chatbot as the type:
+
+    ![Dify create chatbot screen](../../assets/deployment/dify-create-chatbot.png)
+
+1. Click the chatbot you just created to open the chat interface and start interacting with the model:
+
+    ![Dify chat screen](../../assets/deployment/dify-chat.png)