Add jobset_api Python package for PyPI publishing

[rahulmansharamani14]

What type of PR is this?

/kind feature

What this PR does / why we need it:

tl;dr: The JobSet Python SDK is not available on PyPI. This PR creates a new jobset_api package that can be published to PyPI, allowing users to pip install jobset_api to get JobSet API models.

This PR replaces the old auto-generated Python SDK (sdk/python/) with a new lightweight jobset_api Python package at api/python_api/, modeled after Kubeflow Trainer's kubeflow_trainer_api. The package generates only Pydantic models (no
API client, tests, or docs) and is designed to be published to PyPI.

Changes:

• Add VERSION file at repo root for version stamping
• Add api/python_api/ package with pyproject.toml, __init__.py, and README.md
• Add hack/python-api/gen-api.sh generation script (OpenAPI Generator v7.13.0, models-only)
• Add hack/python-api/swagger_config.json with jobset_api package name
• Update Makefile to use new generator, remove test-python-sdk target, update verify and prepare-release-branch
• Update .gitignore and AGENTS.md references
• Remove old sdk/python/ and hack/python-sdk/ directories
• Add GitHub Actions workflow for PyPI publishing via OIDC trusted publishing

Post-merge maintainer action required:

1. Register jobset_api on PyPI
2. Configure OIDC trusted publishing: repo=kubernetes-sigs/jobset, workflow=publish-python-api.yaml, environment=pypi

Which issue(s) this PR fixes:

Fixes #736

Special notes for your reviewer:

This follows the same pattern as Kubeflow Trainer's Python API package (ref).

Does this PR introduce a user-facing change?

Add jobset_api Python package for publishing JobSet API models to PyPI.

[k8s-ci-robot]

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This pull-request has been approved by: rahulmansharamani14
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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rahulmansharamani14 / name: Rahul Mansharamani (0b0d9f2, 0f09546, 1ea28c3, 97df4a2, c3db41c)

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