test(shared): isolate logger test overrides

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/logger.ts

@@ -89,6 +89,11 @@ interface LoggerTestOverrides {
 
 /** @internal test-only seam */
 export function _setLoggerForTesting(overrides: LoggerTestOverrides): void {
+  buffer = []
+  if (flushTimer) {
+    clearTimeout(flushTimer)
+    flushTimer = null
+  }
   if (overrides.filePath !== undefined) logFile = overrides.filePath
   if (overrides.maxSizeBytes !== undefined) maxLogFileSizeBytes = overrides.maxSizeBytes
   if (overrides.maxBackups !== undefined) maxLogFileBackups = overrides.maxBackups