test: stabilize restart rebind transport harness (#163)

[James3014]

Closes the remaining deterministic test-harness gap exposed while qualifying #163 host restart/rebind behavior.

The production MCP/OAuth semantics are unchanged. The #159 restart test intentionally tears down one HTTP server and starts a replacement on the same TCP port. Node/undici may otherwise reuse a dead keep-alive socket from the previous listener and raise ECONNRESET before the replacement server receives the request. The harness now sends Connection: close on its direct HTTP MCP requests so the test measures DevSpace session/rebind behavior rather than client-pool socket reuse.

A/B evidence on clean main:

• original test: reproducible ECONNRESET after the second-generation listener shuts down;
• same source with request-level Connection: close: PASS.

Exact Candidate:

• base 3ca5786ba9d2ad9446e1325d76e48a6424cf9cae;
• target tree e23e2539f151c9c5674268191fb848d639a33c8e;
• one changed file: src/server.test.ts;
• no deletions, no scope escape.

Verification:

• focused P1 Convergence: resolve RECONNECT_REQUIRED session snapshot deadlock after server restart #159/P0: preserve Codex MCP tool availability across deployment restart and OAuth rebind #163 tests: 2/2 PASS;
• full src/server.test.ts: 84/84 PASS;
• npm run typecheck: PASS;
• npm run build: PASS;
• git diff --check: PASS.

This does not weaken stale-session, OAuth replay, token/client/resource binding, or reconnect failure behavior.