chore(tools): Fix rustdoc page parsing in web fetch tool

[JeanMertz]

The HTML section extractor had two failure modes when used against rustdoc-generated pages (docs.rs or local cargo doc output).

First, the section listing was polluted with page-chrome IDs (rustdoc_body_wrapper, rustdoc-toc, rustdoc-modnav, main-content). These were picked up by the ancestor-id fallback in resolve_heading_id because rustdoc wraps the whole page in elements carrying those IDs. A new RUSTDOC_SCAFFOLDING_IDS filter drops them from the listing while still allowing explicit sections=[...] requests to target main-content on non-rustdoc pages.

Second, method and variant documentation was never returned. Rustdoc places the signature heading inside <section id="method.X"> and puts the prose in a sibling <div class="docblock"> outside the section — optionally further wrapped in a <details> toggle. The extractor was walking the section's own children, finding nothing, and returning an empty string. Two new helpers, extract_rustdoc_section and preview_from_section_docblock, resolve the sibling docblock in both the flat and toggle layouts. extract_section_html_from_doc now short-circuits into this path for any <section> target, and the preview walk gains a matching fallback so listing previews agree with extracted bodies.