Add system instruction support and enhance internal API handlers

- Introduced `SystemInstruction` field in `PrepareRequest` and `GenerateContentRequest` for better message parsing.
- Updated `SendMessage` and `SendMessageStream` to handle system instructions in client API calls.
- Enhanced error handling and manual flushing logic in response flows.
- Added new internal API endpoints `/v1internal:generateContent` and `/v1internal:streamGenerateContent`.
- Improved proxy handling and transport logic in HTTP client initialization.

internal/api/server.go

@@ -81,6 +81,8 @@ func (s *Server) setupRoutes() {
 			},
 		})
 	})
+	s.engine.POST("/v1internal:method", s.handlers.Internal)
+
 }
 
 // Start begins listening for and serving HTTP requests.