feat(audience): enforce required properties on Track(IEvent) predefined events (SDK-225)

[ImmutableJeffrey]

Summary

• Flips required value-typed fields on the typed IEvent implementations to nullable — Progression.Status, Resource.Flow, Resource.Amount, Purchase.Value — so an unset caller is distinguishable from the enum / zero default.
• Each ToProperties() throws ArgumentException naming the missing field when a required property is null. The existing try/catch in ImmutableAudience.Track(IEvent) catches, logs, and drops the event — consistent with the never-crash convention on the game thread.
• Required fields already validated (Resource.Currency, Purchase.Currency via ISO 4217, MilestoneReached.Name) stay as-is — no behavior change there.
• Tightens the Track(string) doc comment with a concrete example (purchase without currency / value) so the attribution / conversion reporting risk is explicit, per Natalie's follow-up on the merged PR.
• 7 new tests: a missing-required-field case per predefined event, plus an integration test on Track(IEvent) confirming a malformed Purchase is caught, logged, and dropped (161 → 168).

Rationale

Attribution and conversion pipelines depend on predefined events carrying their required fields. The C# compiler enforces field types but does not enforce that required fields are set to a non-default value, so Track(new Purchase { Currency = "USD" }) was silently shipping a zero-value purchase and breaking downstream reporting.

Web SDK sidesteps this with TS conditional types at compile time. Unity C# cannot — required fields must be enforced at runtime instead. Natalie's explicit direction on Slack: "for Unity maybe we can only call out in the docs & comments for Track(string...) and enforce it in Track(IEvent)."

Field mapping matches the canonical schema in Unity SDK Event Reference v1.

Breaking change

Callers that read a required property directly — e.g. decimal v = purchase.Value; — need purchase.Value!.Value after this change. Construction patterns like new Purchase { Value = 9.99m } are unaffected because of implicit nullable conversion.

Out of scope

• Runtime validation for Track(string) with a predefined event name — docs-only per Natalie's direction.
• Compile-time enforcement analogous to the Web SDK's TS conditional types — not possible in C#.

Linear: SDK-225
Sister ticket: SDK-222 (PR #696)
Parent: SDK-99 Unity SDK v1