fix(validators): harden mcp-name matching (PyPI/NuGet anchoring, comment-form safe) + cargo follow-ups (#1331)

Registry-wide hardening of the `mcp-name:` ownership-token match, plus
the cargo follow-up fixes from the #1330 review — consolidated here per
maintainer request. Rebased onto current `main` (the merged #1330 cargo
commit drops out), so the diff is just the net-new work.

### Changes
- **PyPI/NuGet: boundary-anchored token match.** Replace
`strings.Contains(readme, "mcp-name: "+name)` with the shared
`containsMCPNameToken`, so a README declaring a longer name
(`…/widget-pro`) no longer satisfies an ownership claim for a shorter
prefix (`…/widget`). (Cargo already got this in #1330; NPM is unaffected
— it compares an exact metadata field.)
- **Matcher: treat the HTML comment close as a boundary.** `<!--
mcp-name: NAME-->` / `<!--mcp-name: NAME-->` (any spacing) validate
again — the documented hidden-comment form for PyPI/NuGet — while a
genuine longer name (`…/widget--pro`) still does not.
- **Cargo hardening (review of #1330):** pin scheme+port (not just host)
on the README fetch and redirects; a rate-limited/failed crate-version
existence probe now reports *transient* instead of "not found"; a 403
with the crate present no longer flatly asserts "no README".
- **Docs:** note the token must be followed by a boundary.

### ⚠️ Behavior change for PyPI/NuGet — read before merging
The match is **strictly stricter** (verified by fuzz, 2.3M execs: it can
only flip pass→fail, never fail→pass). After the comment-close fix, the
**only** forms that newly fail are unusual *inline* ones: the token
ending a sentence (`…/my-mcp.`) or glued to a trailing `/`. The
documented forms (own line, in `<!-- … -->`) are unaffected.

**Correcting an earlier claim:** this re-validates on **edits too**, not
just new versions — `edit.go → UpdateServer → ValidateUpdateRequest →
validateRegistryOwnership` runs the token check on any edit of a live
server (only delete-transitions skip it). So an existing PyPI/NuGet
server whose README uses one of the breaking inline forms would fail on
its next publish *or* edit. Given the v0.1 API-freeze posture, this
should land as a deliberate, noted change.

### Testing
`go build`, `vet`, `gofmt`, `golangci-lint` (prod files),
`check-schema`, `validate-examples` all clean. Hermetic + **live**
PyPI/NuGet/cargo suite passes. New: `TestCargoURLAllowed`, a
combined-fixture probe-429→transient case, comment-form matcher cases,
and `FuzzContainsMCPNameToken` (the strictly-stricter property, 2.3M
execs).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/modelcontextprotocol-io/package-types.mdx

@@ -87,6 +87,8 @@ This MCP server executes SQL queries and manages database connections.
 <!-- mcp-name: io.github.username/database-query-mcp -->
 ```
 
+The `mcp-name:` token must be followed by a boundary — a newline, whitespace, an HTML tag, or the comment close `-->`. Keep it on its own line or inside `<!-- … -->`; do not glue it directly to trailing characters such as a sentence-ending period (`…/database-query-mcp.`), which prevents the match.
+
 ## NuGet Packages
 
 For NuGet packages, the MCP Registry currently supports the official NuGet registry (`https://api.nuget.org/v3/index.json`) only.
@@ -125,6 +127,8 @@ This MCP server manages Azure DevOps work items and pipelines.
 <!-- mcp-name: io.github.username/azure-devops-mcp -->
 ```
 
+The `mcp-name:` token must be followed by a boundary — a newline, whitespace, an HTML tag, or the comment close `-->`. Keep it on its own line or inside `<!-- … -->`; do not glue it directly to trailing characters such as a sentence-ending period (`…/azure-devops-mcp.`), which prevents the match.
+
 ## Cargo (Rust) Packages
 
 For Cargo packages, the MCP Registry currently supports the official crates.io registry (`https://crates.io`) only.