{@example} always emits a 3-backtick fence, corrupting examples that contain a fence

_injectExamples wraps the example content in a hard-coded 3-backtick fence:

return '```$language\n$content$trailing```';

Per CommonMark a fenced
code block ends at the first closing fence of at least the opening length, so any
example whose content contains a ``` line truncates the block early and leaks
the remainder of the example into the page as markdown.

Reproduction

example/readme_snippet.md:

// #region snippet
Install it:

```console
dart pub add foo
```
// #endregion snippet

Referenced from a doc comment:

/// {@example /example/readme_snippet.md#snippet}
class Foo {}

Actual output — the block closes at the ```console line's matching fence,
so only Install it: is inside the code block and dart pub add foo plus the
trailing fence escape into the rendered page:

```md
Install it:

```console
dart pub add foo
```

Expected — a fence long enough to contain the content:

````md
Install it:

```console
dart pub add foo
```
````

Suggested fix

Compute the fence length from the content instead of hard-coding 3:

final longest = RegExp(r'^\s*(`{3,})', multiLine: true)
    .allMatches(content)
    .map((m) => m[1]!.length)
    .fold(2, max);
final fence = '`' * max(3, longest + 1);
return '$fence$language\n$content$trailing$fence';

Why it matters

This isn't hypothetical — it hits any example file that is itself markdown, any
Dart example containing a doc comment with a code sample, and any example
containing a string literal with a fence. The failure is silent: no warning, just
broken documentation.