schema_registry/protobuf: fix MESSAGE_REMOVED for removed map fields

[QueLLL]

Protobuf map fields are represented in descriptors as compiler-generated
nested *Entry message types. Redpanda schema registry was comparing those map entry types
as regular nested messages during compatibility checks, so removing a map
field could incorrectly fail with MESSAGE_REMOVED

Skip protobuf map entry types when checking nested message compatibility.
This matches Confluent Schema Registry behavior and fixes map field removal
compatibility for all protobuf map key/value combinations.

Fixes #30374

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v26.1.x
• v25.3.x
• v25.2.x

Release Notes

Bug Fixes

• Schema Registry: fixed protobuf compatibility checks incorrectly failing
  with MESSAGE_REMOVED when a map field is removed.

[CLAassistant]

All committers have signed the CLA.

[vbotbuildovich]

Retry command for Build#84210

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/cluster_linking_e2e_test.py::ShadowLinkingReplicationTests.test_with_restart@{"storage_mode":"tiered_cloud"}

[vbotbuildovich]

CI test results

test results on build#84210

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	ShadowLinkingReplicationTests	test_replication_with_failures	{"storage_mode": "tiered_cloud"}	integration	https://buildkite.com/redpanda/redpanda/builds/84210#019e0676-51be-4294-a8d4-8b49ec5ed43b	19/21	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0050, p0=0.0959, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3917, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkingReplicationTests&test_method=test_replication_with_failures
FLAKY(FAIL)	ShadowLinkingReplicationTests	test_with_restart	{"storage_mode": "tiered_cloud"}	integration	https://buildkite.com/redpanda/redpanda/builds/84210#019e0678-8a4c-40ba-bf51-5f2209d0eabd	8/11	Test FAILS after retries.Significant increase in flaky rate(baseline=0.0079, p0=0.0027, reject_threshold=0.0100)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkingReplicationTests&test_method=test_with_restart
FLAKY(PASS)	WriteCachingFailureInjectionE2ETest	test_crash_all	{"use_transactions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/84210#019e0678-8a4d-4f20-aa35-d778ad017a69	9/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.1029, p0=0.6625, reject_threshold=0.0100. adj_baseline=0.2781, p1=0.1866, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=WriteCachingFailureInjectionE2ETest&test_method=test_crash_all

[nguyen-andrew]

Thanks for reporting and fixing this! LGTM

[nguyen-andrew]

Looks like lint is failing, could you run bazel run //tools:clang_format and amend the existing commit? Thanks!

Need to format

[QueLLL]

Looks like lint is failing, could you run bazel run //tools:clang_format and amend the existing commit? Thanks!

Done, thanks!