[v25.2.x] schema_registry/protobuf: fix MESSAGE_REMOVED for removed map fields

[vbotbuildovich]

Backport of PR #30400
Fixes: #30441,

[vbotbuildovich]

Retry command for Build#84297

please wait until all jobs are finished before running the slash command

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase
tests/rptest/tests/random_node_operations_test.py::RandomNodeOperationsTest.test_node_operations@{"cloud_storage_type":2,"compaction_mode":"adjacent_merge","enable_failures":false,"mixed_versions":true,"with_iceberg":false}

[vbotbuildovich]

CI test results

test results on build#84297

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(FAIL)	RandomNodeOperationsTest	test_node_operations	{"cloud_storage_type": 2, "compaction_mode": "adjacent_merge", "enable_failures": false, "mixed_versions": true, "with_iceberg": false}	integration	https://buildkite.com/redpanda/redpanda/builds/84297#019e1898-5157-4f9e-8180-1a18ab72e9c3	5/11	Test FAILS after retries.Significant increase in flaky rate(baseline=0.0275, p0=0.0000, reject_threshold=0.0100)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=RandomNodeOperationsTest&test_method=test_node_operations

[nguyen-andrew]

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase
tests/rptest/tests/random_node_operations_test.py::RandomNodeOperationsTest.test_node_operations@{"cloud_storage_type":2,"compaction_mode":"adjacent_merge","enable_failures":false,"mixed_versions":true,"with_iceberg":false}

[nguyen-andrew]

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase