[v25.3.x] schema_registry/protobuf: fix MESSAGE_REMOVED for removed map fields

[vbotbuildovich]

Backport of PR #30400
Fixes: #30439,

[vbotbuildovich]

CI test results

test results on build#84296

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	ShadowLinkBasicTests	test_link_creation_checks	{"source_cluster_spec": {"cluster_type": "redpanda"}}	integration	https://buildkite.com/redpanda/redpanda/builds/84296#019e1884-a805-4f2b-a865-f3c85c61522c	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0222, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkBasicTests&test_method=test_link_creation_checks
FLAKY(PASS)	ShadowLinkBasicTests	test_link_creation_checks	{"source_cluster_spec": {"cluster_type": "redpanda"}}	integration	https://buildkite.com/redpanda/redpanda/builds/84296#019e188f-c333-41dc-8c96-4f9fbd48b849	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0222, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkBasicTests&test_method=test_link_creation_checks