test_utils/container_ostream: delegate to fmt::print

[travisdowns]

Follow-up to #30415 addressing review feedback.

Replace the hand-rolled element iteration in the std::vector and
std::unordered_map operator<< overloads in test_utils/container_ostream.h
with fmt::print, which already knows how to format ranges. This requires
elements to be fmt-formattable, so the two test-local types that
previously had only operator<< — test_struct in field_parser_test and
serialization_format in httpd — are converted to the format_to
convention; they pick up the auto-generated fmt::formatter from
base/format_to.h, and the existing operator<< call sites still work via
the blanket overload there.

This changes the logging format because fmt does things differently but this is test-only so I think it's OK.

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
• v25.3.x
• v25.2.x
• v25.1.x

Release notes

• none

[Copilot]

Pull request overview

This PR updates the test-only container streaming utilities to use {fmt}’s built-in range formatting instead of manually iterating container elements, and adjusts a couple of test-local types to be {fmt}-formattable so existing assertion streaming continues to work.

Changes:

• Switch std::vector and std::unordered_map operator<< overloads in test_utils/container_ostream.h to delegate to fmt::print.
• Update expected strings in container_ostream_test.cc to match {fmt}’s default range/map formatting (e.g., [] for vectors, quoted strings in ranges).
• Convert two test-local types (Kafka test_struct, Pandaproxy serialization_format) from operator<< to format_to so they participate in base/format_to.h’s auto fmt::formatter / operator<< bridging.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/v/test_utils/tests/container_ostream_test.cc	Updates golden outputs to match {fmt} range/map formatting and removes the “stream-only element” test case.
src/v/test_utils/container_ostream.h	Implements test-only operator<< for std::vector/std::unordered_map via fmt::print and adds required {fmt} includes.
src/v/test_utils/BUILD	Adds @fmt dependency for the container_ostream test library.
src/v/pandaproxy/parsing/test/httpd.cc	Replaces operator<< for serialization_format with an ADL format_to free function.
src/v/pandaproxy/parsing/test/BUILD	Adds dependencies needed for the new format_to usage (//src/v/base, @fmt).
src/v/kafka/protocol/tests/field_parser_test.cc	Replaces operator<< for test_struct with a format_to member method.
src/v/kafka/protocol/tests/BUILD	Adds dependencies needed for the new format_to usage (//src/v/base, @fmt).

[vbotbuildovich]

CI test results

test results on build#84239

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	ClusterQuotaPartitionMutationTest	test_partition_throttle_mechanism	null	integration	https://buildkite.com/redpanda/redpanda/builds/84239#019e094b-a391-4ec1-879e-edf595fab858	19/21	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0043, p0=0.0824, 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=ClusterQuotaPartitionMutationTest&test_method=test_partition_throttle_mechanism