fix(transport): isHostAllowed mangles IPv6 Host headers

[BryanFRD]

Problem

isHostAllowed splits the Host header on : to strip the port, which corrupts bracketed IPv6 literals:

packages/mcp-core/src/transports/http.ts:36-41

const host = hostHeader.split(':')[0].toLowerCase();
return allowedHosts.some((allowed) => allowed.split(':')[0].toLowerCase() === host);

For Host: [::1]:3000 this yields '['. The allowlist entry [::1]:3000 is mangled the same way, so both sides reduce to '[' and happen to match; [::1] compared against [::1]:3000 does not. Either way the comparison is not doing what it reads as doing.

Why it matters

Any deployment reachable over IPv6 — a dual-stack cluster, ::1 in local testing — gets host checks decided by string accidents rather than by the configured allowlist. Requests are rejected with a 421 (http.ts:232-236) or accepted for the wrong reason, and the failure is hard to read because the mangled value never appears in a log.

Proposed approach

• Parse the header with new URL(\http://${hostHeader}`)and compareurl.hostname`, which handles brackets and the missing-port case.
• Normalise allowlist entries the same way at parse time (parseList) instead of at comparison time.
• Cover [::1]:3000, [::1], localhost:3000, and localhost in packages/mcp-core/src/transports/__tests__/http.test.ts.

Acceptance criteria

• An IPv6 Host header matches an IPv6 allowlist entry and only that entry.
• A malformed Host header is rejected rather than silently reduced to a token that matches something.