fix(appkit): keep SSE generator alive for a grace window so reconnection resumes

[MarioCadenas]

Root cause

Commit 4e923408 ("feat(jobs): add HTTP routes, validation, streaming, and review fixes") added an immediate abortController.abort(...) to both res.on("close") handlers in packages/appkit/src/stream/stream-manager.ts (in _createNewStream and _attachToExistingStream): the moment the last client disconnected (clients.size === 0), the generator was aborted and the stream marked completed.

That broke SSE reconnection. The dev-playground reconnect demo streams a finite generator (counts 1..5, 3s apart) and the client deliberately disconnects after message 2, then reconnects ~3s later with Last-Event-ID. With the immediate abort, the generator was killed during the brief disconnect and the stream became isCompleted, so on reconnect _attachToExistingStream just called res.end() — reconnection delivered nothing and the counter stuck at 2. The same hazard applies to any stream a client momentarily drops.

Fix: disconnect grace window

On last-client disconnect we no longer abort immediately. Instead we schedule a delayed abort via a new _scheduleGraceAbort(streamEntry) helper (shared by both close handlers):

• Default 15s, configurable via StreamConfig.disconnectGraceMs (added to packages/shared/src/execute.ts; default in stream/defaults.ts, wired through the StreamManager constructor like bufferTTL).
• A new disconnectGraceTimer?: NodeJS.Timeout lives on StreamEntry. The timer is unref()'d so it never keeps the process alive.
• When a client (re)attaches in _attachToExistingStream, the pending grace timer is cleared before the client is added — so a reconnect within the window cancels the abort and the live generator keeps running. Reconnection then replays buffered events and resumes new ones.
• The grace timer is also cleared on stream completion/error and in abortAll(), so timers never leak or fire late.

Abandoned-stream cleanup preserved

The original intent of 4e923408 still holds: a truly-abandoned stream (no client reconnects within the window) still has its generator aborted — just disconnectGraceMs later instead of instantly — so background polling loops (e.g. jobs runAndWait) are still stopped.

Tests

Added to packages/appkit/src/stream/tests/stream.test.ts (fake timers):

• Reconnect within the grace window → generator NOT aborted, reconnecting client receives subsequent events (core regression test).
• No reconnect within the window → generator IS aborted after disconnectGraceMs (jobs-cleanup behavior).
• Custom disconnectGraceMs is honored.
• Stream completion clears the pending grace timer (no late abort fires).
• Updated the existing "abort on last-client disconnect" test to assert the abort happens after the grace window rather than immediately.

All appkit tests pass (vitest run --project appkit: 2143 passed), plus pnpm build, pnpm -r typecheck, and Biome on changed files are clean.

Note on sequencing with #438

This touches the same two res.on("close") handlers in stream-manager.ts as PR #438, so the two will need sequencing / a rebase when both land.

This pull request and its description were written by Isaac.