Extract and publish openapi-client-core

[dougborg]

openapi-client-core Extraction and Publishing Plan

• Inventory duplicated logic in katana & stocktrim (retry, pagination, auth, error translation) and prepare migration notes.
• Decide on httpx vs requests (recommend httpx for async/sync interface).
• Define public API: BaseOpenAPIClient, PaginationStrategy, RetryPolicy, RateLimiter interface.
• Write README.md including MIT OSS licensing and code headers.
• Add type hints, and at least minimal unit tests (smoke + fake API server OK).
• Publish first package to TestPyPI and verify install in downstream repo.
• Promote to PyPI under MIT once validation complete.

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• Document design decisions in README and code comments.
• Document any risk areas (e.g., pagination edge cases, rate limiting strategies).
• Tag initial release as v0.1.0.

After Issue #2 is complete, sync and update template projects to use published package.