test surrogate pair encoding boundary

[yagop]

Summary

Add a v2 encoder regression test that places an astral Unicode character across the 2048 UTF-16-code-unit boundary from issue #1335.

Why

The legacy transport's transitive qs encoder split surrogate pairs at 1024-code-unit chunk boundaries and corrupted the following character. V2 uses the platform's native URLSearchParams and is not affected, but this test preserves that behavior if the transport changes later.

Impact

No runtime behavior changes. The test verifies that the exact HTML payload survives form encoding unchanged.

Validation

• bun test test/unit/encode.test.ts
• npm run typecheck:test
• git diff --check