test(amplify_event_enrichment): add unit tests

[ekjotmultani]

Summary

Splits the unit tests for the Event Enrichment packages into their own PR so the core implementation and the tests review independently.

Stacked on #7029. This targets the feat/event-enrichment-client branch (the package PR), so the diff shows only the test files. Once #7029 merges, this will be rebased onto main.

What's here

• amplify_event_enrichment_dart/test/: enriched event serialization, client, global fields manager, session manager
• amplify_event_enrichment/test/: Flutter lifecycle observer

Verification

• amplify_event_enrichment_dart: 41 tests pass (dart test)
• amplify_event_enrichment (Flutter wrapper): 6 tests pass (flutter test)
• Analyzer clean (0 errors, 0 warnings)

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.15%. Comparing base (4485a46) to head (3f201c3).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           feat/event-enrichment-client    #7068      +/-   ##
================================================================
- Coverage                         43.23%   43.15%   -0.08%     
================================================================
  Files                                99      102       +3     
  Lines                              7769     7818      +49     
  Branches                           3401     3423      +22     
================================================================
+ Hits                               3359     3374      +15     
- Misses                             4410     4444      +34     

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