fix(core): handle Anthropic overloaded errors and Unicode encoding issues (#9305)

* fix: handle Anthropic overloaded_error in streaming interfaces

* fix: handle Unicode surrogates in OpenAI requests

Sanitize Unicode surrogate pairs before sending requests to OpenAI API.
Surrogate pairs (U+D800-U+DFFF) are UTF-16 encoding artifacts that cause
UnicodeEncodeError when encoding to UTF-8.

Fixes Datadog error: 'utf-8' codec can't encode character '\ud83c' in
position 326605: surrogates not allowed

* fix: handle UnicodeEncodeError from lone Unicode surrogates in OpenAI requests

Improved sanitize_unicode_surrogates() to explicitly filter out lone
surrogate characters (U+D800 to U+DFFF) which are invalid in UTF-8.

Previous implementation used errors='ignore' which could still fail in
edge cases. New approach directly checks Unicode code points and removes
any surrogates before data reaches httpx encoding.

Also added sanitization to stream_async_responses() method which was
missing it.

Fixes: 'utf-8' codec can't encode character '\ud83c' in position X:
surrogates not allowed

letta/adapters/letta_llm_stream_adapter.py

@@ -37,7 +37,16 @@ class LettaLLMStreamAdapter(LettaLLMAdapter):
         org_id: str | None = None,
         user_id: str | None = None,
     ) -> None:
-        super().__init__(llm_client, llm_config, call_type=call_type, agent_id=agent_id, agent_tags=agent_tags, run_id=run_id, org_id=org_id, user_id=user_id)
+        super().__init__(
+            llm_client,
+            llm_config,
+            call_type=call_type,
+            agent_id=agent_id,
+            agent_tags=agent_tags,
+            run_id=run_id,
+            org_id=org_id,
+            user_id=user_id,
+        )
         self.interface: OpenAIStreamingInterface | AnthropicStreamingInterface | None = None
 
     async def invoke_llm(