fix(ios): prevent keychain precondition crash on unsigned and test builds

[datlechin]

Problem

The iOS Tests CI job has failed on every commit since it was added (back to mid-May), with:

TableProMobile encountered an error (Early unexpected exit, operation never finished bootstrapping. Test crashed with signal trap before establishing connection.)

The result bundle shows passedTests: 0 — the host app crashes at launch before a single test runs. Not specific to any recent PR.

Root cause

KeychainSecureStore.resolveAccessGroup() called preconditionFailure(...) when AppIdentifierPrefix is absent or still the unsubstituted literal $(AppIdentifierPrefix). That is exactly the case in the CI build, which runs with CODE_SIGNING_ALLOWED=NO (no signing means no AppIdentifierPrefix). KeychainSecureStore() is constructed in AppState.init, which runs at app launch, so the precondition trips (SIGTRAP) before tests connect. Signed device/TestFlight builds always have the prefix, so they never hit it.

Fix

• The keychain access group is only needed to share items with the widget extension; an app works fine on its own default keychain without one. resolveAccessGroup() now returns String? and logs a warning instead of crashing when the prefix is unavailable. Queries include kSecAttrAccessGroup only when an access group is present. Signed builds are unchanged (they still get the shared group).
• Hardened the documented "skip launch side-effects under tests" guard: it keyed off XCTestConfigurationFilePath, which is not set under Swift Testing (the suite uses @Suite/@Test). New TestRuntime.isActive also checks XCTestBundlePath and NSClassFromString("XCTestCase"), so analytics, CloudKit sync, widget, and Spotlight side-effects are reliably skipped during tests as the author intended. Centralizes a check that was duplicated in AppState and TableProMobileApp.

Notes

• No user-facing behavior change for signed builds, so no CHANGELOG entry.
• This unblocks the iOS Tests job for the first time; the run on this PR should now actually execute the suite.