[CORE-8999] schema_registry/protobuf: Add test for legacy_map

[BenPope]

The Confluent Schema Registry client renders maps in the expanded form demonstrated by the test, which is considered invalid by libprotobuf >= v25.

The patch required to fix this is: https://github.com/redpanda-data/vtools/pull/3434

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
• v24.3.x
• v24.2.x
• v24.1.x

Release Notes

Bug Fixes

• Schema Registry/Protobuf: Fix a regression with maps.

[rockwotj]

You will need the patch for the bazel build too. There are some examples using single_version_override in MODULE.bazel

[vbotbuildovich]

CI test results

test results on build#61502

test_id	test_kind	job_url	test_status	passed
rptest.tests.datalake.compaction_test.CompactionGapsTest.test_translation_no_gaps.cloud_storage_type=CloudStorageType.S3	ducktape	https://buildkite.com/redpanda/redpanda/builds/61502#0194cbd2-3221-446f-a3c5-f203ec03e574	FLAKY	1/3
storage_single_thread_rpunit.storage_single_thread_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/61502#0194cb83-58b8-4680-a0e4-3092070106b5	FLAKY	1/2

[BenPope]

CI Failure is

• https://redpandadata.atlassian.net/browse/CORE-8990

[vbotbuildovich]

/backport v24.3.x

[vbotbuildovich]

/backport v24.2.x

[vbotbuildovich]

Failed to create a backport PR to v24.3.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-25001-v24.3.x-380 remotes/upstream/v24.3.x
git cherry-pick -x d33def4a19

Workflow run logs.

[vbotbuildovich]

Failed to create a backport PR to v24.2.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-25001-v24.2.x-865 remotes/upstream/v24.2.x
git cherry-pick -x d33def4a19

Workflow run logs.