fix(models): enforce UTC ISO 8601 timestamps and omit null attribution fields in EventRequest

[arman-uc]

What does this PR do?

Fixes #10

When tracking organic events (such as automatic screen tracking via LinkFortyNavigatorObserver or direct calls to LinkForty.trackEvent()), event requests to /api/sdk/v1/event fail with a 400 VALIDATION_ERROR on the server.

There were two causes in EventRequest:

1. Timestamp wasn't UTC: DateTime.now() used local device time without the Z suffix. Strict backend validators (like Zod .datetime()) reject timestamps that don't end in Z.
2. Null fields were sent in JSON: Unpopulated attribution fields (attributedLinkId, attributedClickId, linkOpenedAt, sessionId) were serialized as explicit null values instead of being left out. The backend validator expects optional strings and rejects null.

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What changed?

• Enforced UTC timestamps: Added .toUtc() in EventRequest's constructor so all timestamps are formatted in ISO 8601 UTC with a trailing Z (matching the pattern already used in attribution_context.dart).
• Omitted null fields: Updated EventRequest.toJson() to remove keys with null values before sending, keeping the payload clean and organic events valid.
• Added unit tests: Added tests in test/models/event_request_test.dart to verify UTC Z formatting, timezone conversion, and omission of null attribution fields.

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How was it tested?

• flutter test test/models/event_request_test.dart (all 7 tests passed)
• flutter test (full suite of 118 tests passed)
• flutter analyze (no issues found)