Fix heading anchor links opening in a new tab for non-ASCII headings

What does this MR do and why?

Fixes #602230 (closed). Manually verified on GDK-in-a-box.

After the fix, CJK heading anchors no longer have target="_blank", and clicking them navigates within the same page, like ASCII-only headings:

Root cause

1. Comrak parser includes non-ASCII characters directly in the generated href:

   $ comrak --header-id-prefix 'user-content-' <<< '#### Reproduction Heading - 再現用の見出しです'
   <h4><a href="#reproduction-heading---再現用の見出しです" aria-hidden="true" class="anchor" id="user-content-reproduction-heading---再現用の見出しです"></a>Reproduction Heading - 再現用の見出しです</h4>

   The link then lands in Banzai::Filter::ExternalLinkFilter, where it's treated as external for the reasons that follow.

2. URI.parse() on the non-ASCII fragment raises URI::InvalidURIError:

   $ ruby -e 'require "uri"; URI.parse("#reproduction-heading---再現用の見出しです")'
   /.../lib/uri/rfc3986_parser.rb:84:in `split': URI must be ascii only "#reproduction-heading---\u518D\u73FE\u7528\u306E\u898B\u51FA\u3057\u3067\u3059" (URI::InvalidURIError)
   ...

3. addressable_uri becomes nil

4. target="_blank" gets added since SCHEMES includes nil

Fix

Skip fragment-only links (where href starts with #) at the top of call. (URI::InvalidURIError never fires and addressable_uri never becomes nil.)

🔴 RED → 🟢 GREEN cycle:

1. 70a311b0: Add tests for fragment-only links – 🔴 RED at this point due to the existing bug (job log)
2. 9a1b30dc: Skip fragment-only links – 🟢 GREEN

Notes

I went with the narrowest change here. The check could just as well live in internal_url?, and there may be a broader fix that makes more sense. Open to suggestions.

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