Copilot Chat renders large blank regions after a long multi-turn Agent session

### Description

After keeping a single Copilot Chat Agent session active for many turns and tool calls, the Chat view eventually starts rendering very large blank regions between or inside assistant responses.

The requests continue to complete and tool calls continue running, but the visible output becomes dominated by empty vertical space, requiring excessive scrolling and making the session difficult to use.

The problem appears to depend on session length. It is not present at the beginning of the session, but starts after the session accumulates a long history. The exact threshold is not yet known.

### Steps to Reproduce

1. Open Copilot Chat in Agent mode.
2. Select GPT-5.6 Sol.
3. Continue using the same chat session for many rounds.
4. Let the agent perform a large number of tool calls and intermediate assistant turns.
5. Continue scrolling through or interacting with the same session.
6. Observe that increasingly large blank regions appear in the Chat view.

### Expected Behavior

Only visible assistant content, tool results, and progress items should occupy layout space.

Empty intermediate protocol or tool-call messages should not create large blank regions in the Chat view.

### Actual Behavior

Large blank areas appear in the conversation after the session becomes sufficiently long. Assistant requests still appear to succeed, but visible response text is separated by substantial empty vertical space.

### Reproducibility

Observed in a long-running Agent session. The precise number of turns required to trigger the issue is unknown.

### Environment

- OS: Ubuntu 24.04.4 LTS, Linux x64
- Kernel: 6.17.0-1022-azure
- VS Code: 1.134.0
- VS Code commit: `110a328ea54b42367b803ec53ee0bf52ef26b419`
- VS Code build date: 2026-08-18T18:24:44Z
- GitHub Copilot extension: 0.62.0, bundled build 1
- Copilot completions core: 1.378.1799
- Model: GPT-5.6 Sol
- Mode: Agent / `panel/editAgent`
- Screen reader: Disabled
- GPU rendering: Software/llvmpipe; WebGL unavailable

### Diagnostic Observations

At one point during the affected session:

- Persisted chat session size was approximately 1.02 MB.
- Copilot transcript size was approximately 343 KB.
- A concurrently updated Chat Editing state file had grown to approximately 49 MB.
- The transcript contained approximately 86 `assistant.message` records.
- Approximately 30 assistant records had zero-length `content`.
- Almost all assistant records also contained tool requests.
- No non-empty whitespace-only payloads were found in the persisted chat session or transcript.

This suggests that the blank areas may be caused by Chat view rendering, message virtualization, or handling of empty tool-call assistant messages, rather than the model returning a large string of spaces. This is only a diagnostic hypothesis.

### Relevant Log Excerpt

Conversation and request identifiers have been redacted:

```text
2026-08-21 18:44:16.887 [error] [ChatWebSocketManager] Error for conversation turn
2026-08-21 18:44:16.892 [error] Error: (close code: 1006 Abnormal Closure)
2026-08-21 18:44:16.892 [info] Retrying chat request with default fetcher after: (close code: 1006 Abnormal Closure)
2026-08-21 18:44:16.892 [info] Waiting 1000ms before pinging CAPI to check network connectivity...
2026-08-21 18:44:18.118 [info] CAPI ping successful, proceeding with chat request retry...
2026-08-21 18:44:42.894 [info] ccreq: | success | gpt-5.6-sol | 24768ms | [retry-error-tool/]
2026-08-21 18:44:42.894 [info] [ChatWebSocketManager] WebSocket request failed with successful HTTP fallback (1 consecutive).
```

The same extension log contains several other 1006 Abnormal Closure, internal server error, and successful HTTP fallback sequences. It is not yet clear whether these transport errors cause the blank rendering.

Screenshot: