pandaproxy: fix unmatched-route 404 body shape

[nguyen-andrew]

When a request hits an HTTP path/method that isn't registered with
pandaproxy::server (REST proxy or schema registry), Redpanda returns
Seastar's built-in 404 body {"message": "Not found", "code": 404}.
Schema-registry clients parse a different envelope (error_code rather
than code), so any client logic that inspects 404 bodies breaks on
this shape.

Real pandaproxy handlers already emit the
{"error_code": <int>, "message": "..."} envelope via
pandaproxy::json::error_body. The fix adds a default handler that
emits the same shape and registers it on Seastar's routes table in
pandaproxy::server::start(). Both REST proxy and schema registry
share pandaproxy::server, so the fix lands on both subsystems with a
single registration site.

The three commits:

1. pandaproxy: add default_404_handler for unmatched-route 404 body shape
   New default_404_handler class plus C++ unit test exercising the
   body-shape contract in isolation.
2. pandaproxy: register default_404_handler in server::start()
   Wires the handler so Seastar dispatches to it for any unmatched
   (method, path) combination. Owned via unique_ptr because
   Seastar's add_default_handler does not take ownership (per
   seastar/include/seastar/http/routes.hh:136).
3. tests/rptest: cover unmatched-route 404 shape on REST proxy and SR
   Ducktape assertions on both ports (8082 and 8081) lock in the wire
   format end-to-end and prove the wiring at the cluster level.

Jira: https://redpandadata.atlassian.net/browse/CORE-16251

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

• REST proxy and schema registry now return
  {"error_code": 404, "message": "..."} for unmatched routes,
  matching the envelope clients parse.

[Copilot]

Pull request overview

This PR standardizes the JSON error envelope returned for unmatched routes (unregistered path/method) in Pandaproxy-backed HTTP servers (REST proxy and Schema Registry) by registering a Seastar default handler that emits Pandaproxy’s canonical {"error_code": <int>, "message": "..."} body.

Changes:

• Add a default_404_handler that returns the Pandaproxy/SR-compatible 404 envelope for unmatched routes.
• Register the default handler in pandaproxy::server::start() so it applies to both REST proxy and Schema Registry.
• Add unit + rptest coverage to lock the 404 body shape end-to-end for both ports.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/rptest/tests/schema_registry_test.py	Adds an rptest asserting SR unmatched-route 404 returns error_code envelope.
tests/rptest/tests/pandaproxy_test.py	Adds an rptest asserting REST proxy unmatched-route 404 returns error_code envelope.
src/v/pandaproxy/test/default_404_handler.cc	Adds a C++ unit test validating the default handler’s JSON envelope.
src/v/pandaproxy/test/BUILD	Registers the new C++ unit test target.
src/v/pandaproxy/server.h	Stores the default handler to ensure it outlives Seastar routes.
src/v/pandaproxy/server.cc	Registers the default handler on the Seastar routes table during startup.
src/v/pandaproxy/default_404_handler.h	Introduces the default 404 handler implementation.
src/v/pandaproxy/BUILD	Exposes the new handler header in the pandaproxy library.