fix(dts): check for existence of target path when redirect extension

[Timeless0911]

Summary

Fix edge cases of redirect.dts.extension when the folder and the file have same name.

Now we will check for existence of target path when redirect extension.

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull request overview

This PR fixes a bug in the DTS (TypeScript declaration) redirect functionality by checking for the existence of target paths before adding extensions to import statements. This prevents breaking imports for files that don't exist as TypeScript declarations (e.g., .svg files, type-only packages).

Key changes:

• Refactored the addExtension function to verify target path existence before adding extensions
• Added comprehensive test coverage for edge cases including directory imports and type-only packages
• Updated documentation examples to use .ts config files instead of .js

Reviewed changes

Copilot reviewed 17 out of 19 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/plugin-dts/src/utils.ts	Refactored addExtension function to check path existence before adding extensions, preventing broken imports
tests/integration/redirect/dts.test.ts	Added test expectations for new config module and prebundle-types scenarios
tests/integration/redirect/dtsTsgo.test.ts	Added test expectations for new config module and prebundle-types scenarios
tests/integration/redirect/dts/tsconfig.json	Added prebundle-types path mapping for test coverage
tests/integration/redirect/dts/src/index.ts	Added exports for new test modules (config, prebundle-types)
tests/integration/redirect/dts/src/config.ts	New test file that re-exports from subdirectory
tests/integration/redirect/dts/src/config/load.ts	New test file with loadConfig function
tests/integration/redirect/dts/compile/prebundle-types/	New test package with type-only declarations
tests/integration/redirect/dts-tsgo/	Similar test additions for tsgo variant
website/docs/en/config/lib/experiments.mdx	Updated example config filename from .js to .ts
website/docs/zh/config/lib/experiments.mdx	Updated example config filename from .js to .ts

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