refactor: migrate MParticleIdentityClient to Kotlin (Lane C, part 1)

[nickolas-dimitrakas]

Lane C — identity leaf classes, part 1 (1 of 2 files, 13 of 170 LOC)

Stacked on #777 (Lane E) — merge that one first. Part of the Java→Kotlin migration (PLAN.md).

Converts identity/MParticleIdentityClient. Zero public API diff (bash scripts/api-guard.sh check passes) — this file lives outside com.mparticle.internal, so any diff here is a hard gate failure, not a reviewable one. Each method keeps @Throws(Exception::class) to preserve the throws clause Java callers compile against.

MParticleUserImpl is deliberately NOT in this PR

Lane C's other easy-tier file (157 LOC) turned out to have a real hazard: it has three package-private members (setUserIdentities, setUserIdentity, setUserDelegate/mUserDelegate) that testutils/AccessUtils.java calls directly — from a separate Gradle module.

Kotlin has no package-private visibility. The closest equivalent, internal, is module-scoped and name-mangled on the JVM, so converting these would break that cross-module Java call site outright. Working around the mangling with @JvmName compiles, but makes those three members permanently public in the ABI — a real (if obscure) addition to a frozen-by-default file, which is exactly what this migration is supposed to avoid.

This needs either a coordinated PR that also converts AccessUtils.java/IdentityApi.java, or a restructured test hook. Filing as a follow-up rather than folding it in here or quietly accepting the API growth. Worth adding to PLAN.md's hazard table — package-private Java members with cross-module Java callers are a distinct case from the ones already documented.

Validation

• ./gradlew :android-core:compileDebugKotlin :android-core:compileDebugJavaWithJavac — clean
• ./gradlew :android-kit-base:compileDebugKotlin :android-kit-base:compileDebugJavaWithJavac — clean
• bash scripts/api-guard.sh check — passed, zero diff
• bash scripts/kotlin-migration-progress.sh — 23.3% → 23.3% (rounds the same at one decimal; LOC moved 16,752 → 16,739)
• Local ktlintCheck/unit tests could not run in this environment — Maven Central is rate-limiting dependency resolution (429). The commit hook's own ktlint pass (cached deps) was clean. CI will run the full suite.

Draft while CI runs — flip to ready once green, and once #777 merges so this rebases onto workstation/kotlin-migration directly.

[github-actions]

Kotlin migration progress

This PR migrates 13 lines of Java to Kotlin (+0.0 pp).

Module	Kotlin LOC	Java LOC	Java (facade, staying)	Java left to migrate	Kotlin %
android-core	3417	22251	9241	13010	13.3%
android-kit-base	557	5380	1651	3729	9.4%
kits/rokt/rokt	1122	0	0	0	100.0%
Total	5096	27631	10892	16739	15.6%

Migration goal progress: 23.3% (Kotlin LOC as a share of everything that is not a designated public-API facade.)

126 Java files / 49 Kotlin files in scope.

Largest Java files still to migrate

LOC	File
1514	android-core/src/main/java/com/mparticle/internal/ConfigManager.java
1441	android-kit-base/src/main/java/com/mparticle/kits/KitManagerImpl.java
1089	android-core/src/main/java/com/mparticle/internal/MessageManager.java
1069	android-kit-base/src/main/java/com/mparticle/kits/KitConfiguration.java
858	android-core/src/main/java/com/mparticle/internal/MPUtility.java
840	android-core/src/main/java/com/mparticle/internal/MParticleJSInterface.java
809	android-core/src/main/java/com/mparticle/internal/KitFrameworkWrapper.java
786	android-core/src/main/java/com/mparticle/internal/database/services/MParticleDBManager.java
490	android-kit-base/src/main/java/com/mparticle/kits/mappings/CustomMapping.java
482	android-core/src/main/java/com/mparticle/internal/MParticleApiClientImpl.java
431	android-core/src/main/java/com/mparticle/identity/MParticleIdentityClientImpl.java
375	android-core/src/main/java/com/mparticle/internal/UploadHandler.java
350	android-core/src/main/java/com/mparticle/internal/DeviceAttributes.java
322	android-core/src/main/java/com/mparticle/networking/MParticleBaseClientImpl.java
319	android-core/src/main/java/com/mparticle/internal/MessageHandler.java

[thomson-t]

Rebased onto the updated Lane E branch (#777), which now includes latest main via #789.

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PR Summary

Low Risk
Source-only language migration of a small public interface with explicit JVM throw annotations; behavior and ABI are unchanged per API guard.

Overview
Lane C (part 1) moves the public MParticleIdentityClient interface from Java to Kotlin as part of the planned Java→Kotlin migration, with no intended public API change.

The Java file is removed and replaced by a Kotlin interface with the same four identity HTTP operations (login, logout, identify, modify). Each method keeps @Throws(Exception::class) so Java callers still see throws Exception at compile time. api-guard is expected to stay clean because this type is outside com.mparticle.internal.

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