fix(core): raise LLMEmptyResponseError for empty Anthropic responses (#9624)

* fix(core): raise LLMEmptyResponseError for empty Anthropic responses

Fixes LET-7679: Opus 4.6 occasionally returns empty responses (no content
and no tool calls), causing silent failures with stop_reason=end_turn.

Changes:
- Add LLMEmptyResponseError class (subclass of LLMServerError)
- Raise error in anthropic_client for empty non-streaming responses
- Raise error in anthropic_streaming_interface for empty streaming responses
- Pass through LLMError instances in handle_llm_error to preserve specific types
- Add test for empty streaming response detection

This allows clients (letta-code) to catch this specific error and implement
retry logic with cache-busting modifications.

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

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

* fix(core): set invalid_llm_response stop reason for empty responses

Catch LLMEmptyResponseError specifically and set stop_reason to
invalid_llm_response instead of llm_api_error. This allows clients
to distinguish empty responses from transient API errors.

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

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

---------

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

letta/llm_api/anthropic_client.py

@@ -19,6 +19,8 @@ from letta.errors import (
     LLMAuthenticationError,
     LLMBadRequestError,
     LLMConnectionError,
+    LLMEmptyResponseError,
+    LLMError,
     LLMInsufficientCreditsError,
     LLMNotFoundError,
     LLMPermissionDeniedError,
@@ -957,6 +959,11 @@ class AnthropicClient(LLMClientBase):
 
     @trace_method
     def handle_llm_error(self, e: Exception, llm_config: Optional[LLMConfig] = None) -> Exception:
+        # Pass through errors that are already LLMError instances unchanged
+        # This preserves specific error types like LLMEmptyResponseError
+        if isinstance(e, LLMError):
+            return e
+
         is_byok = (llm_config.provider_category == ProviderCategory.byok) if llm_config else None
 
         # make sure to check for overflow errors, regardless of error type
@@ -1278,7 +1285,7 @@ class AnthropicClient(LLMClientBase):
                 response.stop_reason,
                 json.dumps(response_data),
             )
-            raise LLMServerError(
+            raise LLMEmptyResponseError(
                 message=f"LLM provider returned empty content in response (ID: {response.id}, model: {response.model}, stop_reason: {response.stop_reason})",
                 code=ErrorCode.INTERNAL_SERVER_ERROR,
                 details={