[ibrahim-iqbal]

Summary

Migrates the entire codebase from GSON to kotlinx.serialization, addressing #994.

Changes

• Retrofit: Swapped converter-gson for converter-kotlinx-serialization (Retrofit 2.11.0 built-in support) with Json { ignoreUnknownKeys = true }
• DTOs: Added @Serializable and @SerialName annotations to all data classes (LatestRelease, AssetsItem, all GoogleReaderDTO.* classes, Diff)
• SecurityKey hierarchy: Refactored decode() to use reified type parameters with @PublishedApi internal Json instance — keeps the abstract class approach (per maintainer feedback on feat: replace gson with kotlinx.serialization #997)
• ProviderAPI / DiffMapHolder / DataStoreExt: Replaced Gson with Json and MapSerializer / JsonElement for generic map serialization
• Version catalog: retrofit-gson → retrofit-kotlinx-serialization

Compatibility

• All JSON field names preserved via @SerialName where Gson used @SerializedName
• Existing encrypted SecurityKey data (DES + Base64) remains fully compatible — the JSON layer inside the encryption is format-identical
• ignoreUnknownKeys = true ensures forward compatibility with API responses

Tests

Added SerializationCompatibilityTest that:

• Deserializes hand-written JSON strings matching previous Gson output format
• Verifies round-trip encode/decode for all SecurityKey types, GoogleReaderDTO classes, LatestRelease, and Diff
• Tests Map<String, Diff> serialization compatibility (used by DiffMapHolder cache)

Tests bypass DESUtils (which depends on android.util.Base64) and test the JSON serialization layer directly.

Related

• Closes Migrate to kotlinx.serialization #994
• Supersedes stale feat: replace gson with kotlinx.serialization #997 (addressed all review feedback: kept abstract class, added compatibility tests)

Test plan

• Build compiles: ./gradlew compileFdroidDebugKotlin ✅
• Unit tests pass: ./gradlew testFdroidDebugUnitTest --tests SerializationCompatibilityTest
• Manual: verify app launches and can add/read RSS feeds
• Manual: verify existing encrypted account credentials still decrypt correctly
• Manual: verify GitHub release check still works (LatestRelease deserialization)