fix(agent): defer warning messages after parallel tool results

When parallel tool calls trigger loop detection warnings, the warning
messages (role="user") were inserted between tool result messages
(role="tool"). This breaks the Anthropic API when routed through
OpenAI-compatible proxies (e.g. LiteLLM): the proxy groups consecutive
tool messages into a single user message with tool_result blocks, but
an intervening user warning splits the group, causing orphaned
tool_results and HTTP 400 "tool_use ids without tool_result blocks".

Fix: accumulate warning messages during parallel result processing and
append them after all tool results, preserving the consecutive grouping.

Closes #642

internal/agent/loop.go

@@ -699,7 +699,13 @@ func (l *Loop) runLoop(ctx context.Context, req RunRequest) (result *RunResult,
 
 			// 5. Process results sequentially: emit events, append messages, save to session
 			// Note: tool span start/end already emitted inside goroutines above.
+			// IMPORTANT: warning messages (role="user") must be deferred until AFTER all
+			// tool results are appended. Inserting a user message between tool results
+			// breaks the Anthropic API requirement that all tool_results for a set of
+			// tool_uses must be consecutive (causes "tool_use ids without tool_result"
+			// errors when routed through LiteLLM OpenAI→Anthropic conversion).
 			var loopStuck bool
+			var deferredWarnings []providers.Message
 			for _, r := range collected {
 				// Record tool execution time for adaptive thresholds.
 				toolTiming.Record(r.tc.Name, time.Since(r.spanStart).Milliseconds())
@@ -708,12 +714,14 @@ func (l *Loop) runLoop(ctx context.Context, req RunRequest) (result *RunResult,
 				toolMsg, warningMsgs, action := l.processToolResult(ctx, rs, &req, emitRun, r.tc, r.registryName, r.result, hadBootstrap)
 				messages = append(messages, toolMsg)
 				rs.pendingMsgs = append(rs.pendingMsgs, toolMsg)
-				messages = append(messages, warningMsgs...)
+				deferredWarnings = append(deferredWarnings, warningMsgs...)
 				if action == toolResultBreak {
 					loopStuck = true
 					break
 				}
 			}
+			// Append deferred warnings after all tool results to preserve consecutive grouping.
+			messages = append(messages, deferredWarnings...)
 
 			// Check read-only streak after processing all parallel results.
 			if !loopStuck {