fix: harden SSH tunnel lifecycle

[NotReliable]

Summary

• tolerate ordinary multi-second latency when probing and reusing an SSH forward
• never request remote shutdown for an externally owned backend
• cover concurrent ensure calls and 100 reconnect cycles with constant child counts

Fixes #4144.

Why

The previous one-second request timeout and two-second reuse window could classify a temporarily slow forward as stale. Repeated callers would then replace the forward, increasing the chance that old child processes outlived the connection that created them.

This keeps the existing pending-map serialization, but gives a reusable tunnel a realistic bounded readiness window and makes external-backend ownership explicit in finalization.

Verification

• pnpm --filter @t3tools/ssh test — 27 passed
• pnpm exec vp run --filter @t3tools/ssh typecheck — passed
• pnpm exec vp check — passed (10 pre-existing unrelated React warnings)

The repository-wide vp run typecheck planner was also attempted, but this filtered checkout did not have the unrelated Marketing workspace's astro executable linked. The affected SSH package typecheck above is clean.

Note

Harden SSH tunnel lifecycle to prevent duplicate tunnels and skip remote stop for external backends

• ensureEnvironment now waits up to 15s (was 2s) for an existing tunnel's HTTP endpoint before deciding not to reuse it, preventing spurious duplicate tunnel creation under concurrency.
• The tunnel finalizer skips calling stopRemoteServer when remoteServerKind is 'external', only terminating the local tunnel process.
• SSH_READY_PROBE_TIMEOUT_MS is increased from 1,000ms to 5,000ms; SSH_EXISTING_TUNNEL_READY_TIMEOUT_MS (15,000ms) is introduced as a distinct constant.
• New tests in tunnel.test.ts validate single-tunnel concurrency and stable tunnel counts across 100 reconnect cycles.
• Behavioral Change: external backends no longer have their remote server stopped when a tunnel is closed.

^{Macroscope summarized 39db8ec.}

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