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,
         };