Preserve compaction fields omitted from streamed deltas

[sylvesterkaczmarek]

Fixes #417.

Update each compaction field only when the delta contains it. Check presence through the raw JsonField accessor and keep the existing typed accessor for replacement. Omitted fields retain their previous values; explicit null and empty strings continue to replace them.

Six regression tests cover partial updates in both directions, initially missing fields, explicit null/empty replacements, signature preservation, nonmutation and a complete streamed message converted back to request parameters. Five fail on unchanged main; the explicit-value control passes.

Validation

Both accumulator classes passed all 111 tests on the compatibility Jackson version and all 111 on the published Jackson version. Core Kotlin formatting, detektMain, detektTest, lintKotlin, and git diff --check passed.

Tested on macOS arm64 with JDK 21. The full API/repository suite and Java 8 ecosystem tests were not run. No generated models, public signatures, dependencies or workflows changed. Compaction is beta-only, so no stable counterpart needs this change.

[sylvesterkaczmarek]

Both accumulator test classes pass on the compatibility and published Jackson versions, and core static checks pass. Could a maintainer approve the pending CI workflow?

[tomer-ant]

Closing along with #417. The reasoning is in #417 (comment).