feat: Add isolate bootstrap helper for Amplify platform channels (#5302)

[cadivus]

PR2 of the #5302 stack, on top of #7266. Purely additive — no existing file modified, not exported from the barrel, so single-isolate behaviour is unchanged.

• Adds ensureAmplifyIsolateInitialized(token) / amplifyRootIsolateToken / amplifyIsolateIsInitialized in packages/amplify/amplify_flutter/lib/src/amplify_isolate.dart. Home is amplify_flutter, not amplify_core: RootIsolateToken and BackgroundIsolateBinaryMessenger are Flutter types and amplify_core is deliberately Flutter-free.
• @internal for now — promoting later is additive and non-breaking, while shipping it publicly now would commit us under semver before multi-isolate works end to end (the native SDK still rejects a second configure()). Recommend promoting in the PR that lands the broker + docs + example.
• Two sharp edges the helper absorbs: it is a no-op on the root isolate (registering a background messenger there would replace the root's platform message handler), and amplifyRootIsolateToken returns null on web because RootIsolateToken.instance throws UnsupportedError there rather than returning null.
• 9 tests, 7 of them running in CI on the VM (root/secondary behaviour, idempotency, root no-op); 2 web tests run by hand with --platform chrome since this package's CI is VM-only. Top-level functions rather than AmplifyIsolate.ensureInitialized because amplify_lints bans avoid_classes_with_only_static_members.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.03%. Comparing base (5f163d7) to head (dd1b8fd).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           feat/isolate-spike-bg-messenger    #7269      +/-   ##
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+ Coverage                            40.98%   41.03%   +0.04%     
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  Files                                  121      122       +1     
  Lines                                 8273     8285      +12     
  Branches                              3598     3607       +9     
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+ Hits                                  3391     3400       +9     
- Misses                                4882     4885       +3     

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