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-providers/src/lib.rs

@@ -241,12 +241,14 @@ pub struct Tool {
 pub mod anthropic;
 pub mod databricks;
 pub mod embedded;
+pub mod gemini;
 pub mod oauth;
 pub mod openai;
 
 pub use anthropic::AnthropicProvider;
 pub use databricks::DatabricksProvider;
 pub use embedded::EmbeddedProvider;
+pub use gemini::GeminiProvider;
 pub use openai::OpenAIProvider;
 
 impl Message {