fix: benchmark infrastructure — JVM heap, Liquibase, MockProducer leak

[endrju19]

Summary

Three independent fixes that make `./gradlew :okapi-benchmarks:jmh` complete cleanly. Before these, the JMH run OOMs partway through. All three issues exist on main today; running the benchmark suite without these fixes will fail.

No test or production code is touched — pure benchmark infrastructure.

Fixes

1. Bump JMH JVM heap to `-Xmx8g`

Throughput-mode microbenchmarks call `deliver()` at ~1M ops/s; each call allocates Jackson + Kotlin reflection state for JSON deserialization. At the previous default `-Xmx2g` the allocation rate exceeds GC throughput and OOMs within the first measurement iteration.

2. Pass `-Dliquibase.duplicateFileMode=WARN` as JMH JVM arg

`okapi-postgres.jar` and the fat JMH jar both ship the changelog at the same classpath path (`com/softwaremill/okapi/db/postgres/changelog.xml`). Liquibase 4.x treats duplicate resources as an error by default, which aborts `PostgresBenchmarkSupport` setup. The two files are identical (same jar source on the classpath twice), so `WARN` is safe.

3. Subclass `MockProducer` in `DelivererMicroBenchmark` to `clear()` history after every `send()`

`MockProducer.history` (internal `sent` list) retains every record sent for inspection — there is no eviction. In throughput mode at ~1M ops/s for 30s × forks × iterations that list grew to GBs and OOMed the JVM regardless of heap size. Discarding per call is safe because microbench doesn't inspect what was sent — only timing.

With this fix, `DelivererMicroBenchmark.kafkaDeliver` now produces meaningful numbers (~2.3M ops/s ± <1%) instead of `error > score`.

Files

• `okapi-benchmarks/build.gradle.kts` — JVM args
• `okapi-benchmarks/src/jmh/kotlin/.../DelivererMicroBenchmark.kt` — MockProducer override

Why a separate PR

These are pure infrastructure fixes — completely independent of any specific benchmark or transport implementation. Carved out from PR #46 (KOJAK-82) so they can land on main right away, without waiting for the Kafka deliverBatch (#40) review cycle. PR #46 will then contain only the refreshed JMH numbers.

Test plan

• `./gradlew :okapi-benchmarks:compileJmhKotlin` passes
• Verified locally: full `./gradlew :okapi-benchmarks:jmh` run completes with `BUILD SUCCESSFUL` and no OOM