Dev UI raises a spurious "system instructions were modified between consecutive turns" alert for every multi-agent workflow

> Dev UI issue (prebuilt `adk-web` bundle served by `adk web`) — please transfer to google/adk-web if that is the right home.

The performance check diffs consecutive LLM requests in a session without regard for which agent issued them, so two different agents running one after another in a workflow always trip it. Distinct agents having distinct system instructions is the normal shape of every workflow, not a misconfiguration a developer can fix.

```
cd dev && npx adk web ../samples/workflows/dynamic --port 8123
# app=sequence_route, send "Hello", then click the ⚠ Performance chip on event #4
```

```
Performance Alert: System instructions were modified between consecutive turns in this session.
Preceding Instruction: You are an agent. Your internal name is "city_generator_agent".
Current Instruction: You are an agent. Your internal name is "city_report_agent".
```

Fires on every run of the sample and will fire on essentially every workflow with 2+ LLM agents, training reviewers to ignore a warning that would be meaningful in a single-agent session. The check should compare consecutive requests for the *same* agent, or skip diffs that cross an agent boundary.