fix(guardian): don't throw away transcript when over budget (#16956)

## Description

This PR changes guardian transcript compaction so oversized
conversations no longer collapse into a nearly empty placeholder.

Before this change, if the retained user history alone exceeded the
message budget, guardian would replace the entire transcript with
`<transcript omitted to preserve budget for planned action>`!

That meant approvals, especially network approvals, could lose the
recent tool call and tool result that explained what guardian was
actually reviewing. Now we keep a compact but usable transcript instead
of dropping it all.

### Before
```
The following is the Codex agent history whose request action you are assessing...
>>> TRANSCRIPT START
<transcript omitted to preserve budget for planned action>
>>> TRANSCRIPT END

Conversation transcript omitted due to size.

The Codex agent has requested the following action:
>>> APPROVAL REQUEST START
Retry reason:
Sandbox blocked outbound network access.

Assess the exact planned action below. Use read-only tool checks when local state matters.
Planned action JSON:
{
  "tool": "network_access",
  "target": "https://example.com:443",
  "host": "example.com",
  "protocol": "https",
  "port": 443
}
>>> APPROVAL REQUEST END
```

### After
```
The following is the Codex agent history whose request action you are assessing...
>>> TRANSCRIPT START
[1] user: Please investigate why uploads to example.com are failing and retry if needed.
[8] user: If the request looks correct, go ahead and try again with network access.
[9] tool shell call: {"command":["curl","-X","POST","https://example.com/upload"],"cwd":"/repo"}
[10] tool shell result: sandbox blocked outbound network access
>>> TRANSCRIPT END

Some conversation entries were omitted.

The Codex agent has requested the following action:
>>> APPROVAL REQUEST START
Retry reason:
Sandbox blocked outbound network access.

Assess the exact planned action below. Use read-only tool checks when local state matters.
Planned action JSON:
{
  "tool": "network_access",
  "target": "https://example.com:443",
  "host": "example.com",
  "protocol": "https",
  "port": 443
}
>>> APPROVAL REQUEST END
```

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -129,6 +129,7 @@ async fn wait_for_turn_aborted(
                 EventMsg::TurnAborted(TurnAbortedEvent {
                     turn_id: Some(ref turn_id),
                     ref reason,
+                    ..
                 }) if turn_id == expected_turn_id && *reason == expected_reason
             ) {
                 break;