[FSSDK-12617] Deserialize integers as longs when using Gson

[FarhanAnjum-opti]

Summary

Deserialize integers as longs instead of doubles when using Gson. This allows users to use the resultant Map in downstream serializers (e.g. kotlinx.serialization) more easily.

Changes

• Added ToNumberPolicy.LONG_OR_DOUBLE to GsonConfigParser so integers deserialize as Long instead of Double
• Updated GsonConfigParserTest to verify integer parsing and fix assertion argument order
• Updated OptimizelyJSONWithGsonParserTest to reflect Long expected values

Testing

• Updated existing tests to validate new behavior
• Added integer-specific test case in testFromJson

Related

• Jira: FSSDK-12617
• Fixes GitHub Issue: [BUG] Gson deserialization treats integers as doubles, breaking downstream serializers #622
• Original PR: Deserialize integers as longs when using Gson #623 by @jamiesanson

[muzahidul-opti]

A couple of suggestions by AI

• Consider adding a test case for nested integer values in GsonConfigParserTest.testFromJson — the OptimizelyJSONWithGsonParserTest covers nested objects, but the parser-level test only covers flat values.
• Update the comment in OptimizelyJSON.getValueInternal from Integer, Double to Long, Double (or just Number) since the behavior has changed. (Not in scope of this PR, just noting.)