refactor(plugin-dts): simplify dtsPromises to single dtsPromise

[Timeless0911]

Summary

Refactor dtsPromises array to a single dtsPromise variable in packages/plugin-dts/src/index.ts.
Since there is always only one promise in this environment plugin, using an array was unnecessary and potentially ambiguous.

Related Links

N/A

Checklist

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

[Copilot]

Pull request overview

This PR refactors the DTS promise handling from an array (dtsPromises) to a single variable (dtsPromise) with the assumption that only one promise is used. However, this assumption is incorrect - Rslib supports multiple library formats (e.g., esm and cjs) in a single configuration, where each format is treated as a separate environment that generates its own DTS files.

Key Changes

• Changed dtsPromises array to single dtsPromise variable
• Modified promise collection from array push to direct assignment
• Updated promise resolution to await single promise instead of Promise.all

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