Feature: Enable programmatically pass in api_key besides reading from env

Author: aryasoni98

docs/docs/ai/llm.mdx

@@ -234,6 +234,12 @@ To use the Vertex AI API:
     gcloud auth application-default login
     ```
 
+:::note API Key Not Supported
+
+Vertex AI does **not** support the `api_key` parameter. Vertex AI uses Application Default Credentials (ADC) for authentication instead of API keys. If you provide an `api_key` parameter when using `LlmApiType.VERTEX_AI`, an error will be raised.
+
+:::
+
 Spec for Vertex AI takes additional `api_config` field, in type `cocoindex.llm.VertexAiConfig` with the following fields:
 -   `project` (type: `str`, required): The project ID of the Google Cloud project.
 -   `region` (type: `str`, optional): The region of the Google Cloud project. Use `global` if not specified.

rust/cocoindex/src/llm/gemini.rs

@@ -275,12 +275,15 @@ static SHARED_RETRY_THROTTLER: LazyLock<SharedRetryThrottler> =
 impl VertexAiClient {
     pub async fn new(
         address: Option<String>,
-        _api_key: Option<String>,
+        api_key: Option<String>,
         api_config: Option<super::LlmApiConfig>,
     ) -> Result<Self> {
         if address.is_some() {
             api_bail!("VertexAi API address is not supported for VertexAi API type");
         }
+        if api_key.is_some() {
+            api_bail!("VertexAi API key is not supported for VertexAi API type. Vertex AI uses Application Default Credentials (ADC) for authentication. Please set up ADC using 'gcloud auth application-default login' instead.");
+        }
         let Some(super::LlmApiConfig::VertexAi(config)) = api_config else {
             api_bail!("VertexAi API config is required for VertexAi API type");
         };