test(cli): wait for the server's own listening line and retry on a port race

[ajianaz]

What

Fix the intermittent tole serve test failures ("serve never became healthy") by making the spawn helpers race-free.

Why

serve_http_hardening failed about 1 run in 6 on develop. Reproduced and captured: the harness picks a port by binding 127.0.0.1:0 and closing it; before tole serve binds it, the same ephemeral port can be taken by another test's client socket. The server then exits with binding 127.0.0.1:52603 (in use? change --port): Address already in use and the health poll times out. The same pattern sits in four harnesses; serve_approvals also used fixed ports.

Changes

• tests/common/mod.rs (new): spawn_listening builds the command for a candidate port, pipes its stderr, waits for the server's own "listening on" line (proof this process bound the port, not a stranger answering on it) and, if the process exits first, retries on a fresh port (5 attempts). Stderr is forwarded so it still appears in failure reports.
• serve_http_hardening, mcp_http_hardening, serve_approvals (no more fixed ports) and serve_session_cancel use it.
• A test for the helper itself: an instance that exits before listening is retried and the second one is used.

Testing

• serve_http_hardening + mcp_http_hardening 15 times in a row: 0 failures (before: about 1 in 6). serve_approvals + serve_session_cancel 3 times: green.
• fmt, clippy -D warnings, cargo test --workspace pass; cora review --staged clean.

[github-actions]

🔍 Cora AI Code Review

✅ No issues found. Code looks good!

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