fix: resolve cross-page fragment links in markdown

[jonathanhefner]

⚠️ Disclaimer: This was vibe-coded. I am not a PHP developer, but I am helping https://github.com/modelcontextprotocol/php-sdk set up documentation using phpDocumentor, and I encountered this issue. (See also modelcontextprotocol/php-sdk#232.) I guided Claude through the investigation and self-review, but I am unable to personally judge the fix.

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Markdown links with both a page reference and a fragment anchor like [text](page-b.md#section-two) failed to resolve, producing a warning: "Reference page-b.md#section-two could not be resolved".

Two independent bugs caused this:

1. LinkParser checked str_ends_with($url, '.md') on the full URL including the fragment, so page-b.md#section-two did not match and the .md extension was never stripped. Fix: split the fragment off before the .md check, then reattach it afterward.

2. PageHyperlinkResolver passed the full target reference (including #fragment) to findDocumentEntry(), which always returned null since no document is keyed with a fragment suffix. Fix: split the fragment before the document lookup and append it to the generated URL, matching the pattern already used by DocReferenceResolver.

[wWzZb]

I verified this against the latest upstream main (90594491) using the actual link shape from the PHP SDK docs.

The PHP SDK links from mcp-elements.md to anchors including:

• server-builder.md#manual-capability-registration
• server-builder.md#explicit-element-registration

On current main, guides logs Reference server-builder.md#manual-capability-registration could not be resolved and renders the link text without an <a> element. With this PR applied, the links render as /server-builder.html#... and no unresolved-reference warning is produced.

I also reproduced the old PHP 8.5 / lowest-dependencies failure on the original PR head. It was unrelated to this fix: PHPUnit exited non-zero on 100 dependency deprecations from the lowest Symfony and scrivo/highlight.php versions. Upstream main subsequently addressed that with the Symfony deprecation baseline (b844999e) and a higher highlighter floor (bb8bb544). After merging current main, the PHP 8.5 lowest-dependency unit suite passes (484 tests), and the locked full suite passes (829 tests / 6,032 assertions). PHPStan and the architecture check also pass.

Because I cannot push directly to the contributor branch, I opened a clean, mergeable refresh PR here: jonathanhefner#2

@jonathanhefner, could you merge that refresh PR into this branch? Once it lands, this PR will be current with upstream and ready for a fresh CI run. Maintainers, a review after that refresh would help unblock the PHP SDK documentation links affected by this bug.