fix: reset sessionAgentMap in _resetForTesting for test isolation

- Add sessionAgentMap.clear() to _resetForTesting()
- Prevents test pollution when tests run in parallel in CI

src/features/claude-code-session-state/state.ts

@@ -14,6 +14,7 @@ export function getMainSessionID(): string | undefined {
 export function _resetForTesting(): void {
   _mainSessionID = undefined
   subagentSessions.clear()
+  sessionAgentMap.clear()
 }
 
 const sessionAgentMap = new Map<string, string>()