fix(dts): preserve explicit module extensions

[Timeless0911]

Summary

This PR fixes declaration import redirection for explicit module-kind specifiers. When source code imports a path like ./foo.mjs and declaration emit produces foo.d.mts, redirect.dts.extension now preserves the .mjs specifier instead of rewriting it to the current declaration file's default JS extension.

Checklist

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

[Copilot]

Pull request overview

Fixes @rslib/plugin-dts import redirection so that when a source file explicitly imports a module-kind specifier (e.g. ./foo.mjs) and the emitted declaration target is module-kind (foo.d.mts / foo.d.cts), the redirected import keeps the explicit .mjs/.cjs extension instead of being rewritten to match the current declaration file’s default JS extension.

Changes:

• Update redirectDtsImports to detect when an explicit .mjs/.cjs (and related) specifier should be preserved based on the existence of a sibling .d.mts/.d.cts file.
• Extend redirect integration fixtures to include a .mts module imported via ./foo.mjs.
• Update inline snapshots for both tsc and tsgo integration suites to assert the preserved .mjs specifier and the new emitted foo.d.mts.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/integration/redirect/dtsTsgo.test.ts	Updates snapshots to assert .mjs specifiers are preserved under tsgo redirect scenarios.
tests/integration/redirect/dts/src/index.ts	Adds an explicit ./foo.mjs import and re-exports it to exercise the redirect behavior.
tests/integration/redirect/dts/src/foo.mts	New .mts module fixture that emits foo.d.mts to validate module-kind handling.
tests/integration/redirect/dts.test.ts	Updates snapshots to assert .mjs specifiers are preserved under non-tsgo redirect scenarios.
packages/plugin-dts/src/utils.ts	Implements the preservation logic for explicit module-kind specifiers when corresponding .d.mts/.d.cts outputs exist.