Enable third-party MCP in e2e replay proxy for 1.0.64-1 runtime

The bundled CLI 1.0.64-1 gates third-party MCP servers behind the copilot
user's is_mcp_enabled flag: a null/missing value disables them. The e2e
harnesses register a per-token copilot user that omitted is_mcp_enabled, so
the new runtime skipped non-default MCP servers (e.g. meta-echo). That made
the model report the MCP tool as nonexistent, so the next /chat/completions
request no longer matched a recorded snapshot and the replay proxy returned
HTTP 500 (surfacing as CAPIError: 500 Proxy error), failing every MCP e2e
test that exercises a third-party server.

Default is_mcp_enabled to true when serving /copilot_internal/user for a
registered token (explicit values still win), mirroring the connect-proxy
mock. One shared change fixes the e2e harness for all language SDKs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

test/harness/replayingCapiProxy.ts

@@ -283,9 +283,15 @@ export class ReplayingCapiProxy extends CapturingHttpProxy {
               ? rawAuthHeader
               : undefined;
           const token = authHeader?.replace("Bearer ", "");
-          const userResponse = token
+          const registered = token
             ? this.copilotUserByToken.get(token)
             : undefined;
+          // The CLI gates third-party MCP servers behind the copilot user's
+          // `is_mcp_enabled` flag (a null/missing value disables them). Default
+          // it to true so e2e MCP servers are enabled unless a test opts out.
+          const userResponse = registered
+            ? ({ is_mcp_enabled: true, ...registered } as CopilotUserResponse)
+            : undefined;
           if (userResponse) {
             const headers = {
               "content-type": "application/json",
@@ -1504,6 +1510,7 @@ export type ToolResultNormalizer = {
 export type CopilotUserResponse = {
   login: string;
   copilot_plan?: string;
+  is_mcp_enabled?: boolean;
   endpoints?: {
     api?: string;
     telemetry?: string;