[CLEAN] Synthetic Benchmark PR #30376 - test: unify i18next mocks into centralized helpers

[tomerqodo]

Benchmark PR langgenius#30376

Type: Clean (correct implementation)

Original PR Title: test: unify i18next mocks into centralized helpers
Original PR Description: Consolidate scattered i18next mock implementations across test files into a single source of truth. This reduces duplication and ensures consistent mock behavior.

• Create test/i18n-mock.ts with reusable factory functions
• Update vitest.setup.ts to use the centralized helpers
• Remove redundant mock definitions from 8 test files
• Update testing.md documentation

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Summary

fixes langgenius#30304

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Original PR URL: langgenius#30376