feat: add ctx_size to context length keys for Lemonade server support

- Adds 'ctx_size' field to _CONTEXT_LENGTH_KEYS tuple
- Enables hermes agent to correctly detect context size from custom LLMs
  running on Lemonade server that use this field name instead of the
  standard keys (max_seq_len, n_ctx_train, n_ctx)

agent/model_metadata.py

@@ -203,6 +203,7 @@ _CONTEXT_LENGTH_KEYS = (
     "max_seq_len",
     "n_ctx_train",
     "n_ctx",
+    "ctx_size",
 )
 
 _MAX_COMPLETION_KEYS = (