Session resume fails when tool.execution_complete writes raw multiline content into events.jsonl

## Bug

Resuming a session can fail with:

```text
Failed to resume session: Error: Session file is corrupted
(line 1086: SyntaxError: Unterminated string in JSON at position 8038 (line 1 column 8039))
```

## Environment

- Copilot CLI version: 1.0.24
- OS: Linux
- Session ID affected: `46cb26fd-2fc9-4737-954a-1419bea933ff`

## Root cause

This was **not random file corruption**. The session's `events.jsonl` contained `tool.execution_complete` events that were written with **literal newline characters inside `result.content`**, so a single JSON object was split across multiple physical JSONL lines.

Two corrupted spans were present:

- lines `1086-1088`
- lines `1577-1579`

Both spans reconstructed cleanly into valid single `tool.execution_complete` events when the intermediate line breaks were replaced with escaped `\\n` and the combined string was parsed as JSON.

The affected events contained fetched page content beginning with:

- `Contents of https://www.anthropic.com/pricing:`

So the failure mode appears to be tied to persisted tool output that contains raw multiline content.

## Why this breaks resume

`events.jsonl` expects **one complete JSON object per line**. Once a writer emits literal newlines inside a JSON string value, the JSONL reader later tries to parse the first physical line as a complete JSON object and fails with `Unterminated string in JSON`.

## Evidence

The Copilot CLI log reported:

```text
Failed to parse session 46cb26fd-2fc9-4737-954a-1419bea933ff:
Failed to read JSONL from ~/.copilot/session-state/46cb26fd-2fc9-4737-954a-1419bea933ff/events.jsonl:
Invalid event at line 1086: SyntaxError: Unterminated string in JSON at position 8038
Event: {"type":"tool.execution_complete", ...}
```

External validation showed:

- line `1086` alone is invalid JSON
- line `1086 + "\\n" + 1087 + "\\n" + 1088` parses successfully as a single `tool.execution_complete` event
- same pattern for lines `1577-1579`

## Expected behavior

Session event writing should always escape embedded newlines inside string values so each event stays on one physical JSONL line.

## Suggested fix

1. **Fix on write**: ensure event serialization for `tool.execution_complete` always goes through normal JSON string escaping before appending to `events.jsonl`
2. **Defense on read**: consider a repair path for malformed JSONL where a line looks like the start of an event and subsequent lines can be safely rejoined
3. **Better error message**: include the event type and a hint that multiline tool output may have split a JSONL record

## Related issues

This looks related to the broader session-corruption family, but it appears distinct from the existing U+2028/U+2029 reports because the bad file here is caused by **raw literal newlines in tool output**, not line-separator Unicode characters.

Xác định event writer ghi nối thêm vào events.jsonl và JSONL reader được sử dụng khi tiếp tục các session; kiểm tra cách tool.execution_complete result.content được serialize. Tái hiện lỗi với đầu ra nhiều dòng của tool, sau đó xác minh rằng mỗi event vẫn nằm trên đúng một dòng vật lý và session bị ảnh hưởng có thể được tiếp tục thành công.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.