fix: flag eval snap name

[abelonogov-ld]

Summary

Fix flag eval name

How did you test this change?

Are there any deployment considerations?

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Note

Aligns feature flag evaluation tracing with consistent span naming and attribute/event semantics.

• Use FEATURE_FLAG_SPAN_NAME (evaluation) for flag evaluation spans and attach feature_flag.key, feature_flag.provider.name, and feature_flag.context.id on creation
• Record feature_flag event on the captured evaluation span with optional value, variation index, and in-experiment reason; then end the span
• Sample app: default flagKey set to "my-feature" in MainActivity.kt

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