[BUG] js-debug sessions never surface the debuggee exit code (exitCode missing)

[debugmcpdev]

Bug

list_debug_sessions (and the session snapshot generally) never includes exitCode for JavaScript sessions. Python (debugpy) and the mock adapter surface it via the DAP exited event's exitCode (#220); js-debug launches leave it undefined even after the debuggee demonstrably exits non-zero.

Repro (verified on fix/242-js-launch-barrier-hang @ a8329d6, after #242 was fixed)

1. create_debug_session (javascript)
2. start_debugging tests/fixtures/debug-scripts/js-throws.js with dapLaunchArgs: { stopOnEntry: false } (no breakOnExceptions)
3. Call returns promptly with state: 'stopped' ([BUG] start_debugging hangs past 60s for a fast-crashing JavaScript script #242 fixed), but list_debug_sessions shows no exitCode and no lastStop

Likely area

The debuggee runs in a js-debug child session; the child's exited event body (or the lack of one) doesn't reach SessionManagerCore.handleExited's exitCode capture the way the parent-session path does. Child events are re-emitted verbatim by MinimalDapClient/ChildSessionManager — worth checking whether js-debug even sends exitCode on the child exited event or whether it's dropped in forwarding.

Impact

An agent can't distinguish a crash (exit 1) from a clean exit (exit 0) for JavaScript — the exact contract #220 added for other adapters. This is why the #242 e2e regression guard (tests/e2e/mcp-server-break-on-exceptions.test.ts, js no-option test) asserts state/reason only, unlike its python twin which asserts exitCode non-zero. When this is fixed, strengthen that test to match.