fix: resolve.tsconfigPaths: true not applied in test environment

[MdSadiqMd]

Description

fixes #10054

This PR fixes an issue where Vitest was not preserving user-defined Vite resolve options when creating its test environment. Previously, custom resolve options like extensions, dedupe, preserveSymlinks, and options set by Vite plugins (such as vite-tsconfig-paths) were being discarded.

The root cause was that both VitestPlugin and WorkspaceVitestPlugin were creating a new resolve configuration that only included Vitest's default mainFields and conditions, completely ignoring viteConfig.resolve.

This fix introduces a mergeResolveOptions() helper function that:

1. Preserves all user resolve options from the Vite config
2. Overrides only mainFields and conditions with Vitest's required defaults (critical for test environment)
3. Uses test alias if provided, otherwise preserves user's alias

This ensures that plugins like vite-tsconfig-paths work correctly in Vitest, and any custom resolve configuration is respected in tests while maintaining backward compatibility.

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• Ideally, include a test that fails without this PR but passes with it.
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Tests

• Run the tests with pnpm test:ci.
• Added test/config/test/resolve-options.test.ts - Tests custom resolve options preservation
• Added test/config/test/tsconfig-paths.test.ts - Tests alias resolution
• All existing resolve-related tests pass (32 SSR resolve tests, core resolve tests)

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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[sheremet-va]

This PR fixes an issue where Vitest was not preserving user-defined Vite resolve options when creating its test environment. Previously, custom resolve options like extensions, dedupe, preserveSymlinks, and options set by Vite plugins (such as vite-tsconfig-paths) were being discarded.

This is not how config hook works. Vite accepts a partial config and merges it with user config. With this change, vite will merge the same config twice

[hi-ogawa]

This isn't testing Vite 8's resolve: { tsconfigPaths: true } config.

[MdSadiqMd]

#10054 (comment)