fix(test): add nested beforeEach for mainSessionID test isolation

Previous test was setting mainSessionID to 'main-session-123' and the
next test expected undefined. The outer beforeEach wasn't properly
resetting state between tests in the nested describe block.

Adding a nested beforeEach ensures proper test isolation.

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

@@ -82,6 +82,11 @@ describe("claude-code-session-state", () => {
   })
 
   describe("mainSessionID", () => {
+    beforeEach(() => {
+      // #given - ensure clean state for mainSessionID tests
+      setMainSession(undefined)
+    })
+
     test("should store and retrieve main session ID", () => {
       // #given
       const mainID = "main-session-123"