Agent session resume fails when an event is missing agentDisplayName

## Type of issue

Bug

## Description

Resuming an existing agent session can fail because the session event log cannot be parsed when an event is missing the `agentDisplayName` field.

The UI reports:

`Request session.resume failed ... Failed to read JSONL ... Invalid event at line 2: missing field agentDisplayName`

## Steps to reproduce

1. Open an existing agent session.
2. Send another message to resume the session.
3. Observe that the resume operation fails while reading the session event log.

## Expected behavior

Existing sessions should resume successfully. The event reader should remain compatible with events that do not contain `agentDisplayName`, or migrate/handle those events without making the entire session unusable.

## Screenshot