fix(uitests): stop logging out after every authenticated test

All 7 authenticated-flow XCTestCase classes logged out for real in
tearDown, forcing a real OTP round trip against the production backend
on the next authenticated test. Under CI's back-to-back suite run this
produced ~9 real logins to the same QA phone number in ~9 minutes -
the first few succeeded (~30-40s each) but every one after that failed
to complete within timeout, cascading into 'Login never completed'
across CartCheckoutFlowTests, HomeFiltersFlowTests,
OrderDetailsFlowTests, SavedCardsFlowTests, UserProfileFlowTests.

Removed the per-test logout so the QA account's Keychain-backed session
persists across the whole suite (ensureLoggedIn already no-ops when
already authenticated). Moved the 'must start logged out' guarantee to
the two classes that actually need it - HomeGuestFlowTests and
ProfileLoggedOutFlowTests now force logout themselves right after
app.launch() via a small launch helper, instead of relying on whichever
authenticated class happened to run last.

See decisions/2026-09-11-ui-test-shared-login-session.md.

PediFoodsUITests/CartCheckoutFlowTests.swift

@@ -10,9 +10,7 @@ final class CartCheckoutFlowTests: XCTestCase {
         continueAfterFailure = false
     }
 
-    override func tearDownWithError() throws {
-        XCUIApplication().logoutIfAuthenticated()
-    }
+    // No per-test logout - see decisions/2026-09-11-ui-test-shared-login-session.md.
 
     func testAddProductToCartAndReachCheckout() throws {
         let app = XCUIApplication()