feat(datastore): Refuse configure from a secondary isolate (#5302)

[cadivus]

⚠️ Read the last bullet — the Auth and push guards are BLOCKED, and I stopped rather than ship them unverified. First per-category guardrail for #5302, stacked on #7270.

• AmplifyDataStore.configure now fails fast with a PluginError in an Isolate.spawned isolate, naming the real cause (native DataStore is a single instance per process) and where to go instead. Checked before the config check, because being on the wrong isolate cannot be fixed by supplying a config.
• Detection is ServicesBinding.rootIsolateToken != null — true for an app main isolate and a headless FlutterEngine, so it cannot fire in a single-isolate app. Predicate duplicated from the canonical @internal getter in amplify_flutter (each copy comments the other) because amplify_datastore does not depend on amplify_flutter; no common Flutter-capable package exists, and amplify_core is deliberately Flutter-free.
• 4 new tests, all in CI: guard fires in a secondary isolate with the right message, does not fire on the root isolate (reaches the pre-existing ConfigurationError). Full suite 121/121 green, zero regression.
• 🛑 Auth + push guards blocked. amplifyBackgroundProcessing runs addPlugins([AmplifyAuthCognito(), AmplifyPushNotificationsPinpoint()]) in a headless engine, so both those guards depend on the load-bearing claim that a headless FlutterEngine reports a non-null root-isolate token — and it cannot be CI-verified: the push example has no e2e_android leg and no push package in the repo has an integration_test/ dir. Details in the task report; DataStore is unaffected because it is not in that path.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (feat/isolate-category-guardrails@16686a0). Learn more about missing BASE report.

Additional details and impacted files

@@                         Coverage Diff                         @@
##             feat/isolate-category-guardrails    #7271   +/-   ##
===================================================================
  Coverage                                    ?   32.68%           
===================================================================
  Files                                       ?       34           
  Lines                                       ?     2374           
  Branches                                    ?      915           
===================================================================
  Hits                                        ?      776           
  Misses                                      ?     1598           
  Partials                                    ?        0           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.