Re-add discriminator field to discriminated union JSON schema

[tofarr]

Summary

After PR #1555, the discriminator field was removed from the generated JSON schema for discriminated unions. This caused the OpenAPI documentation tests (tests/agent_server/test_openapi_discriminator.py::test_action_schema_has_discriminator) to fail because Swagger UI needs the discriminator field to properly display discriminated unions instead of showing them as "object | object | object...".

This PR re-adds the discriminator field with the proper mapping in the __get_pydantic_json_schema__ method of DiscriminatedUnionMixin. The test expectation was also updated to include the discriminator field.

Changes:

1. openhands-sdk/openhands/sdk/utils/models.py: Added code to build a discriminator mapping from the $ref schemas and include the discriminator field in the generated JSON schema
2. tests/sdk/utils/test_discriminated_union.py: Updated the test_json_schema_expected test to verify the discriminator field is present

Checklist

• If the PR is changing/adding functionality, are there tests to reflect this?
• If there is an example, have you run the example to make sure that it works?
• If there are instructions on how to run the code, have you followed the instructions and made sure that it works?
• If the feature is significant enough to require documentation, is there a PR open on the OpenHands/docs repository with the same branch name?
• Is the github CI passing?

@tofarr can click here to continue refining the PR

---

Agent Server images for this PR

• GHCR package: https://github.com/OpenHands/agent-sdk/pkgs/container/agent-server

Variants & Base Images

Variant	Architectures	Base Image	Docs / Tags
java	amd64, arm64	eclipse-temurin:17-jdk	Link
python	amd64, arm64	nikolaik/python-nodejs:python3.12-nodejs22	Link
golang	amd64, arm64	golang:1.21-bookworm	Link

Pull (multi-arch manifest)

# Each variant is a multi-arch manifest supporting both amd64 and arm64
docker pull ghcr.io/openhands/agent-server:472e283-python

Run

docker run -it --rm \
  -p 8000:8000 \
  --name agent-server-472e283-python \
  ghcr.io/openhands/agent-server:472e283-python

All tags pushed for this build

ghcr.io/openhands/agent-server:472e283-golang-amd64
ghcr.io/openhands/agent-server:472e283-golang_tag_1.21-bookworm-amd64
ghcr.io/openhands/agent-server:472e283-golang-arm64
ghcr.io/openhands/agent-server:472e283-golang_tag_1.21-bookworm-arm64
ghcr.io/openhands/agent-server:472e283-java-amd64
ghcr.io/openhands/agent-server:472e283-eclipse-temurin_tag_17-jdk-amd64
ghcr.io/openhands/agent-server:472e283-java-arm64
ghcr.io/openhands/agent-server:472e283-eclipse-temurin_tag_17-jdk-arm64
ghcr.io/openhands/agent-server:472e283-python-amd64
ghcr.io/openhands/agent-server:472e283-nikolaik_s_python-nodejs_tag_python3.12-nodejs22-amd64
ghcr.io/openhands/agent-server:472e283-python-arm64
ghcr.io/openhands/agent-server:472e283-nikolaik_s_python-nodejs_tag_python3.12-nodejs22-arm64
ghcr.io/openhands/agent-server:472e283-golang
ghcr.io/openhands/agent-server:472e283-java
ghcr.io/openhands/agent-server:472e283-python

About Multi-Architecture Support

• Each variant tag (e.g., 472e283-python) is a multi-arch manifest supporting both amd64 and arm64
• Docker automatically pulls the correct architecture for your platform
• Individual architecture tags (e.g., 472e283-python-amd64) are also available if needed

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
openhands-sdk/openhands/sdk/utils
models.py	133	44	66%	42–43, 49, 51, 59–63, 86, 92, 134–136, 138, 202, 208–211, 213–214, 217, 220–224, 227–231, 233, 255, 268–269, 271–273, 275, 279, 281, 285
TOTAL	14452	6840	52%

[malhotra5]

LGTM!