fix(grpc): preserve explicit JSON nulls in value mappings

[girishkvs]

Summary

Preserve explicit JSON null values when converting protobuf structured data and metadata.

Fixes #1159.
Fixes #1160.

Reports: @lyingparachute and @RemigiuszDudek.

Changes

File	What
A2ACommonFieldMapper.java	Populate a HashMap instead of using the null-hostile collector, keeping keys with null values. Convert outbound Java nulls to protobuf NULL_VALUE instead of leaving the value kind unset.
A2ACommonFieldMapperTest.java, JSONRPCUtilsTest.java, ToProtoTest.java	Add ten regressions covering nested data and metadata, null list elements, message/task mapping, and JSON-RPC round trips.

Existing absent/empty-Struct and metadata defaults are preserved. No constructor, collection-helper, dependency, or generated-source changes.

Validation

• Clean Java 17 affected-module build: mvn -B -ntp -pl spec-grpc -am clean package.
• 335 tests passed; zero failures, errors, or skips, covering the parent, common, spec, jsonrpc-common, and spec-grpc reactor.
• git diff --check.

The full repository, other JDK versions, and transport/TCK suites were not run locally.