fix(core): add default value for OllamaProvider.default_prompt_formatter (#8911)

When Ollama providers are synced to DB via sync_base_providers(), the
default_prompt_formatter field is lost because ProviderCreate doesn't
include it. When loading from DB and calling cast_to_subtype(), Pydantic
validation fails because the field is required.

This was a latent bug exposed when provider models persistence was
re-enabled in 0.16.2. The field was always required but never persisted.

Adding a default value ("chatml") fixes the issue. The field isn't
actually used in the current implementation - the model_wrapper line
is commented out in list_llm_models_async() since Ollama now uses
OpenAI-compatible endpoints.

Fixes: letta-ai/letta-code#587

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

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

letta/schemas/providers/ollama.py

@@ -24,7 +24,8 @@ class OllamaProvider(OpenAIProvider):
     base_url: str = Field(..., description="Base URL for the Ollama API.")
     api_key: str | None = Field(None, description="API key for the Ollama API (default: `None`).")
     default_prompt_formatter: str = Field(
-        ..., description="Default prompt formatter (aka model wrapper) to use on a /completions style API."
+        default="chatml",
+        description="Default prompt formatter (aka model wrapper) to use on a /completions style API.",
     )
 
     @property