feat: add onError callback for custom error handling

[EliaTolin]

Summary

This PR implements the error handler feature as requested in #4, allowing developers to capture and handle serialization/deserialization errors.

Changes

• Added CacheError class with detailed error information:

  • key: The cache key that failed
  • type: Type of error (serialization, deserializationJson, deserializationFromJson)
  • error: The underlying exception
  • stackTrace: Stack trace for debugging
  • rawData: The data that failed (if available)
  • message: Human-readable error message

• Added CacheErrorType enum to distinguish error types

• Added onError callback parameter to call() method

Usage Example

final data = await RemoteCaching.instance.call<User>(
  'user_profile',
  remote: () async => await fetchUser(),
  fromJson: (json) => User.fromJson(json as Map<String, dynamic>),
  onError: (error) {
    // Log to Sentry, Datadog, etc.
    analytics.logError('cache_error', {
      'key': error.key,
      'type': error.type.name,
      'message': error.message,
    });
  },
);

Backward Compatibility

• No breaking changes
• If onError is not provided, existing silent fallback behavior is preserved
• Errors are still logged in verbose mode

Test plan

• All existing tests pass
• New tests for onError callback invocation
• Tests for CacheError message formatting
• Tests for backward compatibility (no callback)
• Flutter analyze passes with no issues

Closes #4

🤖 Generated with Claude Code

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Codecov Report

❌ Patch coverage is 68.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.30%. Comparing base (5bd88bf) to head (ba136d3).

Files with missing lines	Patch %	Lines
lib/src/remote_caching_impl.dart	42.85%	8 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   88.80%   86.30%   -2.50%     
==========================================
  Files           3        4       +1     
  Lines         125      146      +21     
==========================================
+ Hits          111      126      +15     
- Misses         14       20       +6     

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