fix(cargo): harden README fetch, clarify status handling, strengthen tests (#1330)

Follow-up to #1207, addressing items from the cargo validator review.
**Scoped to cargo + shared helpers — no behavior change for existing
npm/pypi/nuget/oci/mcpb publishers.** Safe to merge and promote.

### Changes
**SSRF hardening of the two-call README retrieval**
- Pin the step-2 README URL to an allowed host (`static.crates.io` in
prod; the base host under test) *before* fetching, so a metadata
response can't steer the validator at an internal/attacker host.
- `CheckRedirect` policy pins every redirect hop to the same allowlist
(the initial URL being pinned isn't enough if an upstream 3xx is
followed).
- Cap the README body with `io.LimitReader` (5 MiB).

> Not publisher-exploitable today (the URL comes from crates.io), so
this is defense-in-depth — but it makes the "host is pinned" guarantee
hold in code rather than rely on crates.io's behavior.

**Clearer status handling** (previously any non-5xx/non-200 collapsed to
"not found")
- **429** → reported as transient/retryable, at both the metadata and
README steps.
- **403** → disambiguated via the crate-version endpoint:
genuinely-missing stays "not found"; *exists-but-no-README* gets an
actionable "add a README with `mcp-name` and republish" message (mirrors
the NuGet validator). This continues the 5xx-vs-403 diagnostic split
@P4ST4S started in the #1207 review — thanks!

**Shared `containsMCPNameToken` helper**
Boundary-anchored ownership-token match (prevents prefix confusion, e.g.
a README declaring `io.github.acme/widget-pro` satisfying a claim for
`io.github.acme/widget`), used here **by the cargo validator only**.
Adopting it in PyPI/NuGet is a separate follow-up because it's a
behavior change for those already-live registries.

**Tests & docs**
Hermetic positive `ServerNameFormats`; combined fixture gains a
version-existence endpoint + cases for 403-missing vs 403-no-README,
429, and prefix-confusion rejection; new foreign-README-host (SSRF)
test; internal test for the helper. Docs: list `crates.io` in the
registry-requirements list + the `Package` model doc.

### Testing
`go build`, `go vet`, full `./internal/validators/...` suite (incl. live
cargo positive), `make check-schema`, `validate-examples` all pass. No
new `golangci-lint` findings.

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

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

internal/validators/registries/mcpname_internal_test.go

@@ -0,0 +1,35 @@
+package registries
+
+import "testing"
+
+// TestContainsMCPNameToken covers the boundary-anchored ownership-token match
+// shared by the PyPI, NuGet, and Cargo validators — in particular that a server
+// name which is a prefix of a longer declared name does not satisfy the match.
+func TestContainsMCPNameToken(t *testing.T) {
+	const name = "io.github.acme/widget"
+
+	cases := []struct {
+		desc    string
+		content string
+		want    bool
+	}{
+		{"exact on its own line", "intro text\nmcp-name: io.github.acme/widget\nmore text", true},
+		{"exact at end of content", "mcp-name: io.github.acme/widget", true},
+		{"followed by HTML tag", "<p>mcp-name: io.github.acme/widget</p>", true},
+		{"followed by space", "mcp-name: io.github.acme/widget is the name", true},
+		{"longer hyphenated name not a match", "mcp-name: io.github.acme/widget-pro\n", false},
+		{"longer dotted name not a match", "mcp-name: io.github.acme/widget.core\n", false},
+		{"longer slashed name not a match", "mcp-name: io.github.acme/widget/sub\n", false},
+		{"absent", "nothing to see here", false},
+		{"different name", "mcp-name: io.github.other/thing\n", false},
+		{"prefix occurrence before a real one still matches", "mcp-name: io.github.acme/widget-pro then mcp-name: io.github.acme/widget\n", true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.desc, func(t *testing.T) {
+			if got := containsMCPNameToken(tc.content, name); got != tc.want {
+				t.Fatalf("containsMCPNameToken(%q, %q) = %v, want %v", tc.content, name, got, tc.want)
+			}
+		})
+	}
+}