chore: import upstream snapshot with attribution

contributing/samples/models/hello_world_apigeellm/README.md

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+# Hello World with Apigee LLM
+
+This sample demonstrates how to use the Agent Development Kit (ADK) with an LLM fronted by an Apigee proxy. It showcases the flexibility of the `ApigeeLlm` class in configuring the target LLM provider (Gemini or Vertex AI) and API version through the model string.
+
+## Setup
+
+Before running the sample, you need to configure your environment with the necessary credentials.
+
+1. **Create a `.env` file:**
+   Copy the sample environment file to a new file named `.env` in the same directory.
+
+   ```bash
+   cp .env-sample .env
+   ```
+
+1. **Set Environment Variables:**
+   Open the `.env` file and provide values for the following variables:
+
+   - `GOOGLE_API_KEY`: Your API key for the Google AI services (Gemini).
+   - `APIGEE_PROXY_URL`: The full URL of your Apigee proxy endpoint.
+
+   Example `.env` file:
+
+   ```
+   GOOGLE_API_KEY="your-google-api-key"
+   APIGEE_PROXY_URL="https://your-apigee-proxy.net/basepath"
+   ```
+
+   The `main.py` script will automatically load these variables when it runs.
+
+## Run the Sample
+
+Once your `.env` file is configured, you can run the sample with the following command:
+
+```bash
+python main.py
+```
+
+## Configuring the Apigee LLM
+
+The `ApigeeLlm` class is configured using a special model string format in `agent.py`. This string determines which backend provider (Vertex AI or Gemini) and which API version to use.
+
+### Model String Format
+
+The supported format is:
+
+`apigee/[<provider>/][<version>/]<model_id>`
+
+- **`provider`** (optional): Can be `vertex_ai` or `gemini`.
+
+  - If specified, it forces the use of that provider.
+  - If omitted, the provider is determined by the `GOOGLE_GENAI_USE_ENTERPRISE` environment variable. If this variable is set to `true` or `1`, Vertex AI is used; otherwise, `gemini` is used by default.
+
+- **`version`** (optional): The API version to use (e.g., `v1`, `v1beta`).
+
+  - If omitted, the default version for the selected provider is used.
+
+- **`model_id`** (required): The identifier for the model you want to use (e.g., `gemini-2.5-flash`).
+
+### Configuration Examples
+
+Here are some examples of how to configure the model string in `agent.py` to achieve different behaviors:
+
+1. **Implicit Provider (determined by environment variable):**
+
+   - `model="apigee/gemini-2.5-flash"`
+
+     - Uses the default API version.
+     - Provider is Vertex AI if `GOOGLE_GENAI_USE_ENTERPRISE` is true; otherwise, Gemini.
+
+   - `model="apigee/v1/gemini-2.5-flash"`
+
+     - Uses API version `v1`.
+     - Provider is determined by the environment variable.
+
+1. **Explicit Provider (ignores environment variable):**
+
+   - `model="apigee/vertex_ai/gemini-2.5-flash"`
+
+     - Uses Vertex AI with the default API version.
+
+   - `model="apigee/gemini/gemini-2.5-flash"`
+
+     - Uses Gemini with the default API version.
+
+   - `model="apigee/gemini/v1/gemini-2.5-flash"`
+
+     - Uses Gemini with API version `v1`.
+
+   - `model="apigee/vertex_ai/v1beta/gemini-2.5-flash"`
+
+     - Uses Vertex AI with API version `v1beta`.
+
+By modifying the `model` string in `agent.py`, you can test various configurations without changing the core logic of the agent.