chore(shared,ui): Fix locale fallback behavior

[alexcarpenter]

Description

Port #7494

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

Summary by CodeRabbit

• Bug Fixes

  • Fixed an issue where data merge operations could inadvertently overwrite existing target values with undefined, ensuring data integrity and preventing unintended value loss during merge processes.

• Tests

  • Enhanced test coverage for localization fallback behavior, including scenarios where translation keys are undefined and where custom translations override defaults, to improve system reliability.

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🦋 Changeset detected

Latest commit: 6f5f4d2

The changes in this PR will be included in the next version bump.

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Project	Deployment	Review	Updated (UTC)
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📝 Walkthrough

Walkthrough

This pull request makes three changes: it adds a changeset file for tracking, modifies the fastDeepMergeAndReplace function in the shared utilities to include an undefined check that prevents overwriting target values with undefined during merge operations, and adds two new test cases to the localization test suite that verify fallback behavior when localization keys are undefined or nested keys are missing, ensuring proper handling of English defaults and provided translations.

Pre-merge checks

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'chore(shared,ui): Fix locale fallback behavior' directly reflects the main changes: fixing undefined value handling in fastDeepMerge and adding locale fallback tests in shared and ui packages.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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📥 Commits

Reviewing files that changed from the base of the PR and between 40e771d and 6f5f4d2.

📒 Files selected for processing (3)

• .changeset/silver-mirrors-repeat.md (1 hunks)
• packages/shared/src/utils/fastDeepMerge.ts (1 hunks)
• packages/ui/src/localization/__tests__/parseLocalization.test.tsx (1 hunks)

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🔇 Additional comments (2)

packages/shared/src/utils/fastDeepMerge.ts (1)

20-20: LGTM - correctly prevents undefined from overwriting target values.

This change ensures that explicitly undefined source values fall back to existing target defaults rather than overwriting them, which aligns with the locale fallback fix objective.

packages/ui/src/localization/__tests__/parseLocalization.test.tsx (1)

41-100: LGTM - comprehensive test coverage for undefined fallback behavior.

The two new tests properly validate that undefined locale values fall back to English defaults while non-undefined values use the provided translations, covering both top-level and nested key scenarios.

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commit: 6f5f4d2