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-cli/src/utils.rs

@@ -138,7 +138,7 @@ pub fn load_config_with_cli_overrides(cli: &Cli) -> Result<Config> {
 
     // Validate provider if specified
     if let Some(ref provider) = cli.provider {
-        let valid_providers = ["anthropic", "databricks", "embedded", "openai"];
+        let valid_providers = ["anthropic", "databricks", "embedded", "gemini", "openai"];
         let provider_type = provider.split('.').next().unwrap_or(provider);
         if !valid_providers.contains(&provider_type) {
             return Err(anyhow::anyhow!(