Converge README.md on v2/main with main's restructured Test servers section #1874

Sub-task of #1821, step 2 of the ordered plan in its description. Blocked on #1866 merging.

Why

main restructured the whole "Test servers" section in #1836 (headings, config table, per-config subsections). v2/main separately corrected one paragraph inside that same region in #1846. Neither change reached the other branch, so the first v2/main → main milestone merge conflicts across the restructured section.

Bringing main's structure onto v2/main now shrinks that to a single hunk — measured in a scratch worktree: 1 conflict hunk, 3 lines, the stale-vs-corrected Mcp-Param-* callout, where you take v2/main's side.

The work

Transplant main's restructured section onto v2/main, folding in the #1846 correction. main's copy still claims:

Mcp-Param-* mirroring is skipped by the SDK in the browser … So calling get_weather from the web client omits Mcp-Param-City and the strict server answers -32020.

That stopped being true when the Inspector took over building the mirrored headers itself — mirroring now works on every client, web included, since the web client's upstream request is issued by the Node backend rather than the browser.

⚠️ Do not copy the whole file

git checkout origin/main -- README.md replaces README.md wholesale and would silently revert #1866's repo-status callout and release-section edits, which exist only on v2/main. Transplant the section, not the file.