fix: multi_agent flaky test (#9314)

* fix(core): handle PermissionDeniedError in provider API key validation

Fixed OpenAI PermissionDeniedError being raised as unknown error when
validating provider API keys. The check_api_key methods in OpenAI-based
providers (OpenAI, OpenRouter, Azure, Together) now properly catch and
re-raise PermissionDeniedError as LLMPermissionDeniedError.

🐛 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix(core): handle Unicode surrogates in OpenAI requests

Sanitize invalid UTF-16 surrogates before sending requests to OpenAI API.
Fixes UnicodeEncodeError when message content contains unpaired surrogates
from corrupted emoji data or malformed Unicode sequences.

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* try to fix

* revert random stuff

* revert some stuff

---------

Co-authored-by: Letta <noreply@letta.com>

tests/integration_test_multi_agent.py

@@ -135,18 +135,50 @@ def roll_dice_tool(client: Letta):
 def test_send_message_to_agent(client: Letta, agent_obj: AgentState, other_agent_obj: AgentState):
     secret_word = "banana"
 
-    # Encourage the agent to send a message to the other agent_obj with the secret string
+    # Send a message to the agent asking it to use the tool
     response = client.agents.messages.create(
         agent_id=agent_obj.id,
         messages=[
             MessageCreateParam(
                 role="user",
-                content=f"Use your tool to send a message to another agent with id {other_agent_obj.id} to share the secret word: {secret_word}!",
+                content=f"IMPORTANT: You MUST use the send_message_to_agent_and_wait_for_reply tool RIGHT NOW to send a message to agent {other_agent_obj.id}. Include the exact secret word '{secret_word}' in your message. Call the tool immediately.",
             )
         ],
     )
 
-    # Get messages from the other agent
+    # FIRST: Verify the sender agent actually called the tool
+    # This catches LLM non-determinism early with a clear error message
+    found_tool_call = False
+    tool_return_message = None
+    target_snippet = f"'agent_id': '{other_agent_obj.id}', 'response': ["
+
+    for m in response.messages:
+        if isinstance(m, ToolReturnMessage):
+            if target_snippet in m.tool_return:
+                found_tool_call = True
+                tool_return_message = m
+                break
+
+    if not found_tool_call:
+        # Print debug info to help diagnose the issue
+        print("\n=== DEBUG: Sender agent did not call the tool ===")
+        print(f"Response messages from agent {agent_obj.id}:")
+        for i, m in enumerate(response.messages):
+            print(f"\nMessage {i} ({type(m).__name__}):")
+            if isinstance(m, ToolReturnMessage):
+                print(f"  Tool return: {m.tool_return}")
+            elif hasattr(m, "content"):
+                print(f"  Content: {m.content}")
+            else:
+                print(f"  {m}")
+        raise AssertionError(
+            f"Sender agent {agent_obj.id} did not call send_message_to_agent_and_wait_for_reply tool. "
+            f"This is likely LLM non-determinism. Check debug output above for what the agent did instead."
+        )
+
+    print(f"\n✓ Tool call verified: {tool_return_message.tool_return[:200]}...")
+
+    # SECOND: Now verify the message arrived at the receiver
     messages_page = client.agents.messages.list(agent_id=other_agent_obj.id)
     messages = messages_page.items
 
@@ -161,46 +193,6 @@ def test_send_message_to_agent(client: Letta, agent_obj: AgentState, other_agent
 
     assert found_secret, f"Secret word '{secret_word}' not found in system messages of agent {other_agent_obj.id}"
 
-    # Search the sender agent for the response from another agent
-    in_context_messages_page = client.agents.messages.list(agent_id=agent_obj.id)
-    in_context_messages = in_context_messages_page.items
-    found = False
-    target_snippet = f"'agent_id': '{other_agent_obj.id}', 'response': ["
-
-    for m in in_context_messages:
-        # Check ToolReturnMessage for the response
-        if isinstance(m, ToolReturnMessage):
-            if target_snippet in m.tool_return:
-                found = True
-                break
-        # Handle different message content structures
-        elif hasattr(m, "content"):
-            if isinstance(m.content, list) and len(m.content) > 0:
-                content_text = m.content[0].text if hasattr(m.content[0], "text") else str(m.content[0])
-            else:
-                content_text = str(m.content)
-
-            if target_snippet in content_text:
-                found = True
-                break
-
-    if not found:
-        # Print debug info
-        joined = "\n".join(
-            [
-                str(
-                    m.content[0].text
-                    if hasattr(m, "content") and isinstance(m.content, list) and len(m.content) > 0 and hasattr(m.content[0], "text")
-                    else m.content
-                    if hasattr(m, "content")
-                    else f"<{type(m).__name__}>"
-                )
-                for m in in_context_messages[1:]
-            ]
-        )
-        print(f"In context messages of the sender agent (without system):\n\n{joined}")
-        raise Exception(f"Was not able to find an instance of the target snippet: {target_snippet}")
-
     # Test that the agent can still receive messages fine
     response = client.agents.messages.create(
         agent_id=agent_obj.id,