[FSSDK-12682] Fix Jackson serializer crash caused by R8 obfuscation

[muzahidul-opti]

Summary

• Add consumer ProGuard rules to keep Jackson PropertyNamingStrategies and PropertyNamingStrategy static field names from being obfuscated by R8

The core-api's JacksonSerializer.getSnakeCaseStrategy() uses reflection (Class.getField("SNAKE_CASE")) to look up Jackson naming strategy fields. When an app includes Jackson on its classpath and R8 obfuscates those field names, the reflection lookup fails with NoSuchFieldException, which crashes the DefaultJsonSerializer.LazyHolder static initializer. All subsequent calls to DefaultJsonSerializer.getInstance() then throw NoClassDefFoundError, preventing any event dispatch (impressions and conversions).

Since proguard-rules.txt is already configured as consumerProguardFiles, these rules are automatically merged into any app that depends on the SDK — no manual ProGuard configuration required by customers.

Both PropertyNamingStrategies (Jackson 2.12+) and PropertyNamingStrategy (Jackson 2.11 and earlier) are covered, matching the two-tier reflection fallback in JacksonSerializer.

Test plan

• Build an app with Jackson on the classpath and R8 minification enabled
• Verify DefaultJsonSerializer initializes without NoClassDefFoundError
• Verify event dispatch (impressions and conversions) works through the default SDK path
• Verify no regression when Jackson is NOT on the classpath (Gson fallback still works)

Issues

• FSSDK-12682
• Related: BUG-8620