feat: Add root-isolate predicate and pin secondary-isolate plugin behaviour (#5302)

[cadivus]

⚠️ Groundwork, not the guard — the guard could not go where the plan said. Stacked on #7269. No behaviour change: one new @internal getter, nothing here consumes it.

• The guards cannot live at this layer. AmplifyCategory.addPlugin (amplify_core/lib/src/category/amplify_categories.dart:91-101) already catches AmplifyAlreadyConfiguredException and registers the plugin anyway, so nothing is left for amplify_flutter to observe. I wrote the guard in hybrid_impl.dart, proved it dead with a test, and reverted it. amplify_core is Flutter-free and cannot tell which isolate it is on, so each native-backed plugin guards in its own native call path instead — see feat(datastore): Refuse configure from a secondary isolate (#5302) #7271 (DataStore) and feat(auth): Refuse addPlugin from a secondary isolate without a messenger (#5302) #7272 (Auth).
• Adds amplifyIsInRootIsolate — true for an app main isolate and a headless FlutterEngine, false only inside Isolate.spawn. Cannot false-positive on the root isolate.
• Pins the category-layer swallow described above in CI, plus: a pure-Dart plugin still configures fine in a secondary isolate, and the root isolate still tolerates an already-configured native SDK (hot-restart path). 15 tests, all in CI.
• Deferred follow-up — push guard. Not in this stack. It needs an integration_test/ harness for the amplify_push_notifications_pinpoint example, an e2e_android leg (neither exists today), and a test-only native hook to invoke the background service and report amplifyIsInRootIsolate from inside the real headless engine. Native work with its own testing plan; the guard must not ship on static analysis alone.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 41.58%. Comparing base (dd1b8fd) to head (16686a0).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           feat/isolate-bootstrap-helper    #7270      +/-   ##
=================================================================
+ Coverage                          41.03%   41.58%   +0.54%     
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  Files                                122      124       +2     
  Lines                               8285     8193      -92     
  Branches                            3607     3587      -20     
=================================================================
+ Hits                                3400     3407       +7     
+ Misses                              4885     4786      -99     

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