fix(ts): surface incomplete sandbox run instead of silent success

[sgirones]

Problem

Sandbox.run() (the method behind runCommand) streams POST /api/v1/processes/run and only sets exitCode when it observes an exit_code/signal event. When the SSE stream ends without a terminal event, it fell through to:

return { exitCode: -1, stdout: "", stderr: "" };

…and never threw. Callers got a successful CommandResult with empty output for a command that never actually ran.

How it shows up

Running a sandbox benchmark against a dev endpoint, every exec returned "success" with empty stdout after a fixed ~48s. Tracing it end-to-end:

• The in-VM daemon executes node -v in ~11ms (direct and via the dataplane's own http_proxy).
• During the 48s, the executor dataplane saw zero connections and no process was ever spawned (Failed to send signal: No processes spawned).
• The exec request goes to the sandbox-proxy host (sandbox.<env>), which couldn't route to the executor; it closed the stream after a timeout without any exit_code.
• The SDK returned { exitCode: -1, stdout: "" } → the harness counted 100/100 "ok" for commands that never executed.

The underlying routing problem is dev infrastructure (separate issue), but the SDK masking it as success is a real correctness bug.

Fix

Track whether a terminal (exit_code/signal) event arrived; if the stream ends without one, throw SandboxConnectionError instead of returning a misleading success.

This matches the canonical behavior already in the repo's Rust CLI (crates/cli/src/commands/sbx/exec.rs), which treats a missing exit event as failure (exit_code.unwrap_or(1) → non-zero → error).

Tests

• New unit test in tests/sandbox.test.ts reproducing the unreachable case (run stream with no exit_code) → asserts run() rejects.
• tsc --noEmit clean; full suite green (189 tests, unit + integration).

Out of scope

• The dev sandbox-proxy ↔ executor routing/latency (infra).
• Signal-exit code convention: TS uses -signal while the Rust CLI uses 128 + signal; worth aligning separately.

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