feat(provider): add gemini provider (#21)

Author: bzp2010

docs/internals/llm-types.md

@@ -140,6 +140,8 @@ For OpenAI-compatible providers, the concrete definition can stay very small. Th
 
 `OpenAIDef` remains hand-written because it needs its own default quirk profile, while `DeepSeek` is the first macro-generated provider in the new stack.
 
+`GoogleDef` currently stays on Gemini's OpenAI-compatible endpoint in the staged implementation, so it is also expressed as a small provider definition instead of a native Gemini transform. That keeps the migration incremental while leaving room for a later native Gemini provider.
+
 `AnthropicDef` is the first hand-written non-OpenAI-compatible provider in the new stack. It combines a custom `ChatTransform` with `NativeAnthropicMessagesSupport` so Anthropic Messages requests can bypass the hub format when the caller already speaks the native protocol.
 
 ### Runtime provider instances

src/gateway/providers/gemini.rs

@@ -0,0 +1,33 @@
+use crate::gateway::providers::macros::provider;
+
+provider!(GoogleDef {
+    display_name: "gemini",
+    base_url: "https://generativelanguage.googleapis.com/v1beta/openai",
+    chat_path: "/chat/completions",
+    auth: api_key_header("x-goog-api-key"),
+});
+
+#[cfg(test)]
+mod tests {
+    use super::GoogleDef;
+    use crate::gateway::{provider_instance::ProviderAuth, traits::ProviderMeta};
+
+    #[test]
+    fn google_def_uses_compatible_gemini_endpoint_and_auth_header() {
+        let provider = GoogleDef;
+        let headers = provider
+            .build_auth_headers(&ProviderAuth::ApiKey("gemini-key".into()))
+            .unwrap();
+
+        assert_eq!(provider.name(), "gemini");
+        assert_eq!(
+            provider.default_base_url(),
+            "https://generativelanguage.googleapis.com/v1beta/openai"
+        );
+        assert_eq!(
+            provider.build_url(provider.default_base_url(), "gemini-2.5-flash"),
+            "https://generativelanguage.googleapis.com/v1beta/openai/chat/completions"
+        );
+        assert_eq!(headers["x-goog-api-key"], "gemini-key");
+    }
+}

src/gateway/providers/mod.rs

@@ -1,10 +1,12 @@
 pub mod anthropic;
 pub mod deepseek;
+pub mod gemini;
 pub mod macros;
 pub mod openai;
 
 pub use anthropic::AnthropicDef;
 pub use deepseek::DeepSeek;
+pub use gemini::GoogleDef;
 pub use openai::OpenAIDef;
 
 use crate::gateway::{error::Result, provider_instance::ProviderRegistry};
@@ -13,6 +15,7 @@ pub fn default_provider_registry() -> Result<ProviderRegistry> {
     let builder = ProviderRegistry::builder()
         .register(OpenAIDef)?
         .register(AnthropicDef)?
+        .register(GoogleDef)?
         .register(DeepSeek)?;
     Ok(builder.build())
 }
@@ -27,6 +30,7 @@ mod tests {
 
         assert_eq!(registry.get("openai").unwrap().name(), "openai");
         assert_eq!(registry.get("anthropic").unwrap().name(), "anthropic");
+        assert_eq!(registry.get("gemini").unwrap().name(), "gemini");
         assert_eq!(registry.get("deepseek").unwrap().name(), "deepseek");
         assert!(registry.get("missing").is_none());
     }