Why

This adds the remote_compaction_v2 client path so remote compaction can run through the normal Responses stream and install a context_compaction item that trigger a compaction.

The goal is to migrate some of the compaction logic on the client side

We keeps the v2 transport behind a feature flag while letting follow-up requests reuse the compacted context instead of falling back to the legacy compaction item shape.

What changed

• add ResponseItem::ContextCompaction and refresh the generated app-server / schema / TypeScript fixtures that expose response items on the wire
• add core/src/compact_remote_v2.rs to send compaction through the standard streamed Responses client, require exactly one context_compaction output item, and install that item into compacted history
• route manual compact and auto-compaction through the v2 path when remote_compaction_v2 is enabled, while keeping the existing remote compaction path as the fallback
• preserve the new item type across history retention, follow-up request construction, telemetry, rollout persistence, and rollout-trace normalization
• add targeted coverage for the feature flag, context_compaction serialization, rollout-trace normalization, and remote-compaction follow-up behavior

Verification

• added protocol tests for context_compaction serialization/deserialization in protocol/src/models.rs
• added rollout-trace coverage for context_compaction normalization in rollout-trace/src/reducer/conversation_tests.rs
• added remote compaction integration coverage for v2 follow-up reuse and mixed compaction output streams in core/tests/suite/compact_remote.rs