fix: add Claude Haiku 4.5 model to MODEL_LIST and add defensive prefix stripping (#8908)

- Added `claude-haiku-4-5-20251001` and `claude-haiku-4-5-latest` to MODEL_LIST
  in anthropic.py to fix context window lookup for the newly released model
- Added prefix stripping in anthropic_client.py to handle cases where the
  model name incorrectly includes the `anthropic/` provider prefix

Fixes the production error:
anthropic.NotFoundError: Error code: 404 - model: anthropic/claude-haiku-4-5-20251001

Fixes #8907

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

Co-authored-by: letta-code <248085862+letta-code@users.noreply.github.com>
Co-authored-by: datadog-official[bot] <datadog-official[bot]@users.noreply.github.com>
Co-authored-by: Kian Jones <11655409+kianjones9@users.noreply.github.com>

letta/llm_api/anthropic_client.py

@@ -447,8 +447,14 @@ class AnthropicClient(LLMClientBase):
         else:
             max_output_tokens = llm_config.max_tokens
 
+        # Strip provider prefix from model name if present (e.g., "anthropic/claude-..." -> "claude-...")
+        # This handles cases where the handle format was incorrectly passed as the model name
+        model_name = llm_config.model
+        if "/" in model_name:
+            model_name = model_name.split("/", 1)[-1]
+
         data = {
-            "model": llm_config.model,
+            "model": model_name,
             "max_tokens": max_output_tokens,
             "temperature": llm_config.temperature,
         }

letta/schemas/providers/anthropic.py

@@ -93,6 +93,16 @@ MODEL_LIST = [
         "name": "claude-3-5-haiku-latest",
         "context_window": 200000,
     },
+    # 4.5
+    {
+        "name": "claude-haiku-4-5-20251001",
+        "context_window": 200000,
+    },
+    # 4.5 latest
+    {
+        "name": "claude-haiku-4-5-latest",
+        "context_window": 200000,
+    },
     ## Opus 4.5
     {
         "name": "claude-opus-4-5-20251101",