[CORE-15194] schema_registry: add subject query param to GET /schemas/ids/{id}

[nguyen-andrew]

Add an optional subject query parameter to the GET /schemas/ids/{id} endpoint. This allows specifying the context for schema lookup by extracting the context from the provided subject name (e.g., :.myctx:mysubject).

Fixes CORE-15194

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 adds an optional subject query parameter to the GET /schemas/ids/{id} endpoint to enable context-aware schema lookups. When provided, the subject name (which can include context in the format :.context:subject) is used to determine the context for retrieving the schema, rather than always using the default context.

Changes:

• Modified the endpoint handler to parse and use the optional subject parameter to extract context information
• Updated the Python test client to support passing the subject parameter
• Added comprehensive test coverage verifying the new parameter works correctly with default and non-default contexts
• Updated API documentation to describe the new query parameter

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/v/pandaproxy/schema_registry/handlers.cc	Added logic to parse the optional subject query parameter, extract context from it, and use the context when looking up schemas by ID
tests/rptest/tests/schema_registry_test.py	Updated test client method to accept subject parameter and added comprehensive test case covering various scenarios
src/v/pandaproxy/api/api-doc/schema_registry.json	Added documentation for the new subject query parameter

[nguyen-andrew]

Force push to rebase on latest dev.

[vbotbuildovich]

CI test results

test results on build#79806

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
ScalingUpTest	test_fast_node_addition	null	integration	https://buildkite.com/redpanda/redpanda/builds/79806#019c06fb-10a4-47cf-a3ce-73980f76c3be	FLAKY	19/21	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0206, p0=0.3402, 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=ScalingUpTest&test_method=test_fast_node_addition

[pgellert]

I think the behaviour is a bit trickier here unfortunately:

# Register the same schema in two contexts
% curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" --data '{"schema": '"$(cat ~/tasks/avro-refs/address.avsc | jq -Rs .)"', "schemaType": "AVRO"}' http://localhost:8081/subjects/:.prod:Ad
dress/versions
{"id":1,"version":1,"guid":"a3d4c656-76ec-775d-35a1-1de29d031a17","schemaType":"AVRO","schema":"{\"type\":\"record\",\"name\":\"Address\",\"fields\":[{\"name\":\"street\",\"type\":\"string\"},{\"name\":\"city\",\"type\":\"string\"}]}"}%

% curl -X POST -H "Content-Type: application/vnd.schemaregistry.v1+json" --data '{"schema": '"$(cat ~/tasks/avro-refs/address.avsc | jq -Rs .)"', "schemaType": "AVRO"}' http://localhost:8081/subjects/:.shared:Address/versions
{"id":1,"version":1,"guid":"a3d4c656-76ec-775d-35a1-1de29d031a17","schemaType":"AVRO","schema":"{\"type\":\"record\",\"name\":\"Address\",\"fields\":[{\"name\":\"street\",\"type\":\"string\"},{\"name\":\"city\",\"type\":\"string\"}]}"}%


# Query the schemas

% curl "http://localhost:8081/schemas/ids/1?subject=:.shared:"
{"subject":":.shared:Address","version":1,"guid":"a3d4c656-76ec-775d-35a1-1de29d031a17","schemaType":"AVRO","schema":"{\"type\":\"record\",\"name\":\"Address\",\"fields\":[{\"name\":\"street\",\"type\":\"string\"},{\"name\":\"city\",\"type\":\"string\"}]}","ts":1769688260232,"deleted":false}%                                                                                                                                                                                                                                    

% curl "http://localhost:8081/schemas/ids/1?subject=:.prod:"
{"subject":":.prod:Address","version":1,"guid":"a3d4c656-76ec-775d-35a1-1de29d031a17","schemaType":"AVRO","schema":"{\"type\":\"record\",\"name\":\"Address\",\"fields\":[{\"name\":\"street\",\"type\":\"string\"},{\"name\":\"city\",\"type\":\"string\"}]}","ts":1769688254717,"deleted":false}%                                                                                                                                                                                                                                    

% curl "http://localhost:8081/schemas/ids/1?subject=Address"
{"subject":":.prod:Address","version":1,"guid":"a3d4c656-76ec-775d-35a1-1de29d031a17","schemaType":"AVRO","schema":"{\"type\":\"record\",\"name\":\"Address\",\"fields\":[{\"name\":\"street\",\"type\":\"string\"},{\"name\":\"city\",\"type\":\"string\"}]}","ts":1769688254717,"deleted":false}%                                                                                                                                                                                                                                    

% curl "http://localhost:8081/schemas/ids/1?subject=:.:"
{"error_code":40403,"message":"Schema 1 not found"}%

% curl "http://localhost:8081/schemas/ids/1?subject=:.prod:NotAddress"
{"error_code":40403,"message":"Schema 1 not found"}%

% curl "http://localhost:8081/schemas/ids/1?subject=:.shared:NotAddress"
{"error_code":40403,"message":"Schema 1 not found"}%

I think we might need to treat the subject parameter differently depending on whether it contains only a context (empty subject) or a real subject.

To be honest, we might be able to get away with partial support of the parameter, by throwing if it contains a real subject, and not just a context. But if we can implement it fully, that would be great.

[pgellert]

Sorry, I should have been clearer in the ticket, but can you please also implement this for GET /schemas/ids/{id}/versions and GET /schemas/ids/{id}/schema too, in addition to GET /schemas/ids/{id}?