Add Google Gemini provider support

- Add GeminiProvider with streaming and native tool calling
- Support gemini-2.5-pro, gemini-2.0-flash, gemini-1.5-pro/flash models
- Model-specific context window detection (1M-2M tokens)
- Message conversion: assistant -> model role mapping
- System messages extracted to system_instruction field
- Tool schema conversion with functionCall/functionResponse parts
- SSE streaming with JSON array buffer parsing
- 8 unit tests for conversion and parsing logic
- Register provider in g3-core and validate in g3-cli

crates/g3-core/src/lib.rs

@@ -708,6 +708,18 @@ impl<W: UiWriter> Agent<W> {
                     16384 // Conservative default for other Databricks models
                 }
             }
+            "gemini" => {
+                // Gemini models - use provider's context_window_size()
+                if let Some(ctx_size) = provider.context_window_size() {
+                    debug!(
+                        "Using context window size {} from Gemini provider",
+                        ctx_size
+                    );
+                    ctx_size
+                } else {
+                    1_000_000 // Default for Gemini models
+                }
+            }
             _ => config.agent.fallback_default_max_tokens as u32,
         };
 

crates/g3-core/src/provider_registration.rs

@@ -57,6 +57,7 @@ pub async fn register_providers(
     register_openai_providers(config, providers_to_register, &mut registry)?;
     register_openai_compatible_providers(config, providers_to_register, &mut registry)?;
     register_anthropic_providers(config, providers_to_register, &mut registry)?;
+    register_gemini_providers(config, providers_to_register, &mut registry)?;
     register_databricks_providers(config, providers_to_register, &mut registry).await?;
 
     // Set default provider
@@ -162,6 +163,27 @@ fn register_anthropic_providers(
     Ok(())
 }
 
+/// Register Gemini providers from configuration.
+fn register_gemini_providers(
+    config: &Config,
+    providers_to_register: &[String],
+    registry: &mut ProviderRegistry,
+) -> Result<()> {
+    for (name, gemini_config) in &config.providers.gemini {
+        if should_register(providers_to_register, "gemini", name) {
+            let gemini_provider = g3_providers::GeminiProvider::new_with_name(
+                format!("gemini.{}", name),
+                gemini_config.api_key.clone(),
+                Some(gemini_config.model.clone()),
+                gemini_config.max_tokens,
+                gemini_config.temperature,
+            )?;
+            registry.register(gemini_provider);
+        }
+    }
+    Ok(())
+}
+
 /// Register Databricks providers from configuration.
 ///
 /// This is async because OAuth authentication requires async operations.