React: Preserve discriminated union prop values in metadata extraction

[s-robertson]

What I did

Fix react-component-meta prop extraction for discriminated unions where the same prop has different types across union members.

Previously, the extractor retained the first prop symbol while deduplicating union members, causing later literal values to be omitted. It now uses TypeScript's merged property type for serialization while preserving the original symbol for documentation, defaults, declarations, and source filtering.

Added a regression test confirming an optional discriminant with "a" and "b" variants includes both values.

AI assistance: OpenAI Codex was used to investigate, implement, and test this change. I reviewed and submitted the result.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

No browser-based manual testing is necessary. This change only affects TypeScript component metadata extraction and is covered by the regression test.

A maintainer can verify it by running:

1. cd code/renderers/react
2. node ../../../node_modules/vitest/vitest.mjs run --config vitest.config.ts src/componentManifest/componentMeta/componentMetaExtractor.props.test.ts
3. Confirm all tests pass, including merges same-named prop types across union members.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Documentation updates are not applicable because this fixes existing experimental extraction behavior without changing its API or configuration.

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Declare whether manual QA will be needed for this PR during the next release, through qa:needed or qa:skip

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook publish.yml --field pr=<PR_NUMBER>

[github-actions]

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🚫	PR is not labeled with one of: ["qa:needed","qa:skip","qa:success"]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 8821720a-80d9-4b52-a782-a6739b7ad6a3

📥 Commits

Reviewing files that changed from the base of the PR and between 9164b22 and 7079faf.

📒 Files selected for processing (2)

• code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.props.test.ts
• code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.ts

🚧 Files skipped from review as they are similar to previous changes (2)

• code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.props.test.ts
• code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.ts

Included review availability: Your plan includes up to 10 reviews per rolling hour; 9 remain after this review.

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Walkthrough

Changes

Union Prop Serialization

Layer / File(s)	Summary
Prop type override code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.ts	extractPropItem can serialize an explicitly resolved property type.
Union property resolution and coverage code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.ts, code/renderers/react/src/componentManifest/componentMeta/componentMetaExtractor.props.test.ts	Union properties use types resolved from the complete union. Tests cover optionality, inherited properties, never exclusions, and combined enum values.

Possibly related PRs

• storybookjs/storybook#35913: Both changes modify type serialization logic in componentMetaExtractor.ts, including union handling.

Merge Risk: ⚪ Minimal · up to 7079f

This localized fix preserves all literal values for discriminated-union props during metadata extraction and includes a regression test; no actionable merge-blocking risk remains beyond normal checks and review.

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