[v25.3.x] dl/coordinator: add default formatter for errc

[vbotbuildovich]

Backport of PR #31003

• Command: git cherry-pick -x a9b2926
• Commits backported: 1
• Conflicts resolved: 1
• Commits skipped (already on target): 0
• Backport branch: ai-backport-pr-31003-v25.3.x-1783022853

Conflict details

• a9b2926 (src/v/datalake/coordinator/types.h, src/v/datalake/coordinator/types.cc): The source commit added an unknown-value fallback to a format_to(errc, ...) free function in types.h, but that format_to-based formatter only exists on dev — v25.3.x still formats errc via operator<<(std::ostream&, const errc&) declared in types.h and defined in types.cc (the operator<<→format_to migration is a separate, un-backported commit). Resolved by keeping the target branch's operator<< declaration in types.h and applying the commit's actual intent — gracefully formatting unknown/future numeric errc values — to the operator<< definition in types.cc. Mirrored dev's exhaustive-switch + post-switch fallback by converting the per-case breaks to early returns and adding return o << fmt::format("errc::unknown({})", static_cast<int16_t>(errc)); after the switch, preserving -Wswitch exhaustiveness checking.

[vbotbuildovich]

CI test results

test results on build#86683

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FAIL	src/v/storage/tests/storage_e2e_single_thread	src/v/storage/tests/storage_e2e_single_thread		unit	https://buildkite.com/redpanda/redpanda/builds/86683#019f2475-7db8-4862-bdfc-cdc851e4776a	0/1

[vbotbuildovich]

Retry command for Build#86683

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/data_migrations_api_test.py::DataMigrationsApiTest.test_creating_and_listing_migrations