No heartbeat event during LLM-streamed tool-argument construction; watchdogs can't distinguish working from hung

### Summary

When the model streams a large tool-call argument value (e.g., a 20KB `file_text` for `create`), the event stream is functionally silent for 10–30 minutes between `tool.execution_start` and `tool.execution_complete`. A stall watchdog watching the event stream cannot distinguish "model is productively building a large output" from "session has hung."

### Repro

```ts
const session = await client.createSession({ /* ... */ });
session.on((evt) => console.log(evt.type, evt.data));
await session.sendAndWait({
prompt: "Create a file at ./report.json containing a JSON document " +
"with at least 20KB of structured analysis data. Use the create tool.",
});
```

Observe the event stream between `tool.execution_start` (with empty `arguments`) and `tool.execution_complete` (with the full argument). Empirically, no events fire for the duration of argument construction.

### Expected

A heartbeat event during argument streaming — byte count, token count, or even a simple `tool.argument_progress` ping — so consumers can tell a working session from a hung one.

### Actual

`tool.execution_start` fires once, then silence for the duration of LLM argument streaming, then `tool.execution_complete` fires once when the full tool invocation finishes (which includes both arg construction AND tool execution).

`assistant.streaming_delta` exists with cumulative byte counts (`totalResponseSizeBytes`) and fires during response streaming, but it has not been verified whether it continues to fire during the tool-argument construction phase. **This would be useful to clarify**: if `streaming_delta` does cover that phase, the gap is consumer-side documentation; if it doesn't, the gap is upstream.

### Evidence (SDK source)

`nodejs/src/generated/session-events.ts`: the `SessionEvent` union includes:
- `assistant.streaming_delta` (`AssistantStreamingDeltaData.totalResponseSizeBytes`) — described as "Streaming response progress with cumulative byte count"
- `assistant.reasoning_delta`, `assistant.message_delta` — response-side
- `tool.execution_progress`, `tool.execution_partial_result` — fire AFTER `execution_start` for tools that emit stdout (`bash`, `powershell`)

There is no `tool.argument_progress` or equivalent event specifically scoped to in-flight tool-argument streaming.

### Consumer impact

Consumers with long-running tool calls must either tune stall watchdogs to ceilings that mask real failures (15+ minutes) or route artifact completion through filesystem polling instead of session events.

### Suggested fix

Either:
- **Confirm + document** that `assistant.streaming_delta` continues firing during in-flight tool-argument construction. If so, this is a doc-only fix.
- **Emit a `tool.argument_progress` event** during LLM-streamed argument construction carrying byte/token count or a small delta. Even one event every ~10 seconds would let consumers tell working sessions from hung ones.

### Related

- #590 — "Long-running prompt suddenly hangs forever with no error or exception" — describes the user-visible symptom. The missing in-flight heartbeat is one root cause: consumers can't distinguish hang-in-progress from work-in-progress without it.
- #1273 — `session.disconnect()` cooperative — the recovery path when this stall does happen is also broken; the two issues compound.

### Environment

```
- SDK: @github/copilot-sdk@0.3.0
- CLI: @github/copilot@1.0.45
- Node: 22 LTS
- OS: Windows 11
- Model: claude-sonnet-4-6
```

Commencez par nodejs/src/generated/session-events.ts et les définitions des événements assistant.streaming_delta, tool.execution_start et tool.execution_complete. Vérifiez si streaming_delta se déclenche pendant la construction en cours des arguments d’un outil ; le travail est considéré comme terminé si ce comportement est documenté ou si un événement de progression délimité est défini et émis afin que les consommateurs puissent distinguer un travail actif d’un blocage.

Rédigé par le modèle d'indexation à partir du texte de l'issue.