Add presented offering context to paywall events

[rickvdl]

Summary

• Include placement_identifier, targeting_revision, and targeting_rule_id in paywall events under a presented_offering_context nested object
• Field is omitted when no placement or targeting data exists
• Consistent with iOS and Android implementations

Related PRs

• iOS: Add placement and targeting rule to paywall events purchases-ios#6476
• Android: Add placement and targeting context to paywall events purchases-android#3253

Test plan

• Unit tests for all context permutations (full, placement-only, targeting-only, none, empty string)
• JSON round-trip serialization tests
• Integration test verifying context flows from offering to event payload
• Manual testing with targeting rules and placements in demo app