fix: reject a non-string protocolVersion, and drop the test double casts

Copilot review follow-ups:

- `applyProtocolVersion` took the value straight from an unvalidated JSON
  body and went to `RegExp.test`, which coerces — `123` and `true` both
  matched the token pattern and reached `Transport.setProtocolVersion` with
  the wrong type. It now takes `unknown` and checks `typeof === "string"`
  first, with cases for the coercible values.
- Replace `as unknown as Transport` in the new session fixtures with a
  `makeTransport()` helper returning a structurally valid `Transport`, so an
  interface change stays visible to the tests.
- Drop `as unknown as typeof fetch` in the new transport test —
  `vi.fn<typeof fetch>()` is already the right type.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0119LaPQ3v4NyBK3ZTdM9j84

core/mcp/remote/node/remote-session.ts

@@ -69,11 +69,14 @@ export class RemoteSession {
    *
    * Ignores anything that isn't a plausible version token so a client can't
    * push arbitrary bytes into an upstream header, and re-applies only on
-   * change (the value rides every `/api/mcp/send`).
+   * change (the value rides every `/api/mcp/send`). The value arrives from an
+   * unvalidated JSON body, so the type is checked rather than assumed —
+   * `RegExp.test` would otherwise coerce a `123` or `true` into a "matching"
+   * string and hand the wrong type to the transport.
    */
-  applyProtocolVersion(version: string | undefined): void {
+  applyProtocolVersion(version: unknown): void {
     if (
-      version === undefined ||
+      typeof version !== "string" ||
       version === this.appliedProtocolVersion ||
       !RemoteSession.PROTOCOL_VERSION_PATTERN.test(version)
     ) {