refactor: reduce log verbosity by adjusting log levels

- Change most INFO logs to DEBUG level for operational messages
- Keep stderr messages at INFO level
- Rename errorHandler to stdErrorHandler in StdioClientTransport
- Add debug logging for JSON message deserialization

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransport.java

@@ -251,7 +251,7 @@ public class WebFluxSseServerTransport implements ServerMcpTransport {
 				.then(Mono.fromRunnable(() -> sessions.remove(sessionId)));
 		}).toList()))
 			.timeout(Duration.ofSeconds(5))
-			.doOnSuccess(v -> logger.info("Graceful shutdown completed"))
+			.doOnSuccess(v -> logger.debug("Graceful shutdown completed"))
 			.doOnError(e -> logger.error("Error during graceful shutdown: {}", e.getMessage()));
 	}
 

mcp-spring/mcp-spring-webmvc/src/main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransport.java

@@ -353,7 +353,7 @@ public class WebMvcSseServerTransport implements ServerMcpTransport {
 				sessions.remove(sessionId);
 			});
 
-			logger.info("Graceful shutdown completed");
+			logger.debug("Graceful shutdown completed");
 		});
 	}