/compact cannot recover a session after the CAPI Responses payload reaches the 5 MB limit`

### Describe the bug

### Description

When a Copilot CLI session reaches the 5 MB CAPI Responses request limit, normal prompts fail as expected—but `/compact` also fails. This leaves no way to reduce the active context while preserving and continuing the session. Basically this kills my session and I cannot continue in it which is a huge kill in productivity and flow.

### Error

> The request is too large to send through CAPI Responses. Try shortening the conversation or prompt. (5.0 MB request; 5.0 MB limit)
>

### Environment

- GitHub Copilot CLI: `1.0.78-0`
- OS: macOS `26.5.2`
- Architecture: `arm64`


### Affected version

1.0.78-0

### Steps to reproduce the behavior

## Steps to reproduce

1. Continue an interactive Copilot CLI session until its request context reaches 5 MB.
2. Submit another prompt.
3. Observe the request-size error.
4. Run `/compact`.
5. Observe that compaction cannot complete either.

### Expected behavior

## Expected behavior

`/compact` should remain usable after the normal request limit is reached, perhaps by truncating or chunking context locally before requesting a summary. The complete transcript should remain stored locally.

Ideally, Copilot CLI should also compact automatically before reaching an unrecoverable size.

### Actual behavior

Both regular prompts and `/compact` are blocked. The session history still exists, but the session cannot be continued; the only practical workaround is starting a new session and manually transferring context.

### Additional context

Additional detail

This occurs when the UI reports only approximately 69–70% context usage (about 1.1M context shown), rather than near 100%.

The context meter appears to track token capacity, while CAPI applies a separate 5 MB serialized-request limit. The CLI neither warns about this lower effective limit nor compacts before reaching it.

I have attached a screenshot showing the session at 70% context when the failure occurred.

Start by reproducing the failure in an interactive Copilot CLI session when the CAPI Responses request reaches the 5 MB limit, then run /compact. Trace the compaction and request-size handling paths; done means /compact can reduce context and continue the existing session, while the complete transcript remains stored locally.

Written by the indexing model from the issue text.