fix(agents): prevent Anthropic 400 'Incorrect role information' error

Add validateAnthropicTurns() to merge consecutive user messages that can
occur when steering messages are injected during streaming. This prevents
the API from rejecting requests due to improper role alternation.

Changes:
- Add validateAnthropicTurns() function in pi-embedded-helpers.ts
- Integrate validation into sanitization pipeline in pi-embedded-runner.ts
- Add user-friendly error message for role ordering errors
- Add comprehensive tests for the new validation function

src/agents/pi-embedded-runner.ts

@@ -100,6 +100,7 @@ import {
   pickFallbackThinkingLevel,
   sanitizeGoogleTurnOrdering,
   sanitizeSessionMessagesImages,
+  validateAnthropicTurns,
   validateGeminiTurns,
 } from "./pi-embedded-helpers.js";
 import {
@@ -1291,7 +1292,9 @@ export async function compactEmbeddedPiSession(params: {
               sessionManager,
               sessionId: params.sessionId,
             });
-            const validated = validateGeminiTurns(prior);
+            // Validate turn ordering for both Gemini (consecutive assistant) and Anthropic (consecutive user)
+            const validatedGemini = validateGeminiTurns(prior);
+            const validated = validateAnthropicTurns(validatedGemini);
             const limited = limitHistoryTurns(
               validated,
               getDmHistoryLimitFromSessionKey(params.sessionKey, params.config),
@@ -1714,7 +1717,9 @@ export async function runEmbeddedPiAgent(params: {
               sessionManager,
               sessionId: params.sessionId,
             });
-            const validated = validateGeminiTurns(prior);
+            // Validate turn ordering for both Gemini (consecutive assistant) and Anthropic (consecutive user)
+            const validatedGemini = validateGeminiTurns(prior);
+            const validated = validateAnthropicTurns(validatedGemini);
             const limited = limitHistoryTurns(
               validated,
               getDmHistoryLimitFromSessionKey(params.sessionKey, params.config),