fix: bounds check in compact boundary loop

When preserve_recent_messages == 0, raw_keep_from equals messages.len(),
causing index out of bounds when accessing session.messages[k].

Added k >= session.messages.len() check to prevent panic.

Reason: Compaction with preserve_recent_messages=0 triggered OOB access
when checking for tool-use/tool-result pair preservation at boundary.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: 2 people

rust/crates/runtime/src/compact.rs

@@ -128,7 +128,7 @@ pub fn compact_session(session: &Session, config: CompactionConfig) -> Compactio
         // is NOT an assistant message that contains a ToolUse block (i.e. the
         // pair is actually broken at the boundary).
         loop {
-            if k == 0 || k <= compacted_prefix_len {
+            if k == 0 || k <= compacted_prefix_len || k >= session.messages.len() {
                 break;
             }
             let first_preserved = &session.messages[k];