Fix {@ escaping in markdown to preserve PHPDoc annotations like {@inheritDoc}

[nicodevs]

Fixes #749 - PHPDoc {@inheritDoc} annotations in markdown code blocks were being corrupted.

Given the following Markdown:

---
extends: _layouts.main
section: body
---

# Test Page with PHP code

```php
/**
 * {@inheritDoc}
 */
function test() {}
```

The @ escaping ('@' => "{{'@'}}") was turning {@inheritDoc} into {{{'@'}}inheritDoc} - three opening braces, which is invalid Blade syntax. The build failed with a ParseError: syntax error, unexpected token "{".

The fix: Add '{@' => '{@' to the replacement array. Since strtr prioritizes longer matches first, {@ is matched before @, preserving the original sequence.

Added test case for {@inheritDoc} in code blocks.