Discriminated Union Support

[THEjacob1000]

Internal change only

This pull request adds support for overriding the request and response type definitions (TypeDef) used in endpoint metadata, enabling advanced schema features such as discriminated unions. It also includes new tests to ensure these overrides are correctly used, and updates the TypeScript client code generation and its tests to handle union types.

Submitter Checklist

Check only those that apply to this change.

• Self-reviewed code, removed extraneous comments, debug logging, checked code style
• No change to users after merge (off-by-default configuration, feature flag, alt URL, etc.) and can be disabled if issues arise (if this is not the case we may need stakeholder signoff)
• Changes are documented (README, wiki, etc)
• Automated tests added or existing tests cover the new functionality or changes
• Manually tested changes
• Metrics added to track the usage/performance
• I am confident I can revert this change
• Database migrations are reversible without data loss (if applicable)
• Performance impact is acceptable (if applicable)
• Any new dependencies are justified or have been approved (if applicable)
• This is NOT a high risk change (if it is, please follow the high-risk change process)

Testing Evidence

What evidence supports that you have tested this change?

• Test cases cover the new functionality or changes
• Screenshots or logs of the changes (if applicable)
• Performance benchmarks (if applicable)
• Storybook cases (if applicable)

Reviewer Checklist

Note: if this PR affects authentication or payments please seek a secondary tester.

• I am ok with code style and functionality
• I have personally tested the feature
• My review was not rushed due to time constraints