Ollama max_tokens settings (#17025)

- Support `available_models` for Ollama
- Clamp default max tokens (context length) to 16384.
- Add documentation for ollama context configuration.

crates/assistant/src/assistant_settings.rs

@@ -135,6 +135,7 @@ impl AssistantSettingsContent {
                                         Some(language_model::settings::OllamaSettingsContent {
                                             api_url,
                                             low_speed_timeout_in_seconds,
+                                            available_models: None,
                                         });
                                 }
                             },
@@ -295,7 +296,7 @@ impl AssistantSettingsContent {
                             _ => (None, None),
                         };
                         settings.provider = Some(AssistantProviderContentV1::Ollama {
-                            default_model: Some(ollama::Model::new(&model)),
+                            default_model: Some(ollama::Model::new(&model, None, None)),
                             api_url,
                             low_speed_timeout_in_seconds,
                         });