fetch: ask for native markdown through Accept content negotiation #5198

Problem

fetch_url in src/fetch/src/mcp_server_fetch/server.py sends no Accept header, so a site that can serve markdown directly through content negotiation sends HTML, which is then simplified. When a site does return text/markdown, the tool labels it as content that "cannot be simplified to markdown" and prefixes the raw-content notice.

Proposal

Prefer text/markdown in the Accept header and return a markdown response as-is, without the raw-content prefix. This is an enhancement, so whether it fits the reference server is a maintainer's call.

Prior work

• feat(fetch): use HTTP content negotiation to request native markdown #4052 by @sidney

The PR(s) above targeted the pre-refactor code and are closed as part of the backlog triage in #4875; this issue is where the work continues, with the PR(s) as a prototype. Thank you to their authors.