userPromptTransformed hook is skipped for steering messages

[Filed by Copilot on behalf of @bghgary]

Describe the bug

When a session receives a user message while the agent is already processing, the message is delivered to the model as steering, but the userPromptTransformed hook is not invoked. Hooks that inject standing instructions therefore apply to ordinary and queued turns but not to corrections or redirections delivered mid-turn.

Affected version

GitHub Copilot CLI 1.0.80

Steps to reproduce the behavior

1. Configure a userPromptTransformed command hook that adds a unique marker to modifiedTransformedPrompt and logs each invocation.
2. Start a session and send an ordinary prompt. Confirm that the hook runs and the marker reaches the model context.
3. Start a long-running tool call, then send another user message before the tool call completes.
4. Inspect the session events and hook log.

The second message is recorded with delivery: "steering", but it has no userPromptTransformed hook events, the hook log has no corresponding invocation, and the marker is absent from the model context.

Expected behavior

userPromptTransformed should run before every user message is added to a model request, including messages delivered while the session is processing.

Additional context

In a clean Windows x64 reproduction, the ordinary prompt invoked the hook while the in-flight steering prompt did not. In a larger sample of 984 user-message turns, the injected marker was present on 96.6% of idle deliveries and 100% of queued deliveries, but only 4.9% of steering deliveries; the hook's own 1,859-entry log contained no failures.

The installed implementation has separate paths: ordinary turn construction calls userPromptTransformed, while an in-flight send() queues the message through addImmediateMessagesIfProcessing() and runImmediatePromptPreRequest() later appends it directly to the model request without applying the hook.

Start by tracing addImmediateMessagesIfProcessing() and runImmediatePromptPreRequest(), then compare that path with the ordinary turn construction path that invokes userPromptTransformed. Reproduce the steering-message scenario from the issue and verify that the hook runs, emits an event, and places the marker in the model context before the message is added to the request.