## Why
Remote compaction v2 consumes a normal Responses stream, but that
compaction-specific stream consumer dropped the `response.completed` id.
As a result, the `responses_websocket_response_processed` lifecycle
notification was emitted for normal turn sampling but not after a v2
remote compaction response was fully processed.
## What changed
- Return the completed response id alongside the v2 `context_compaction`
output item.
- After v2 compacted history is installed, send `response.processed`
through the same websocket session when the feature is enabled.
- Add websocket regression coverage for a remote compaction v2 request
followed by `response.processed`.
## Verification
- `cargo test -p codex-core --test all
responses_websocket_sends_response_processed_after_remote_compaction_v2
-- --nocapture`
- `cargo test -p codex-core
collect_context_compaction_output_accepts_additional_output_items --
--nocapture`
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