fix(editor): render strikethrough on links (#15109)

**Issue**

Strikethrough on a link doesn't render. The toolbar button highlights
but no line
appears (#15106).

**Solution**

affine-link hardcoded text-decoration: none in the override it passes to
affineTextStyles, which clobbered the decoration computed from
strike/underline.
Removing it fixes the render; plain links still show no underline
because
affineTextStyles returns none by default.

**Result**

Strikethrough and underline render on links again. Added an e2e test: a
plain link
stays undecorated, a struck link renders line-through, red before the
fix and green
after.

fix #15106

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Fixed link text-decoration styling to properly support strikethrough
and other text formatting when applied to links.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/inlines/link/src/link-node/affine-link.ts

@@ -160,7 +160,6 @@ export class AffineLink extends WithDisposable(ShadowlessElement) {
     const linkStyle = {
       color: 'var(--affine-link-color)',
       fill: 'var(--affine-link-color)',
-      'text-decoration': 'none',
       cursor: 'pointer',
     };