Introduce LanguageModelToolUse::raw_input (#29322)

This is to enable alternative streaming solutions at the application
layer. I'm not sure we really should have performed parsing of the input
at this layer. Either way I want to experiment with streaming approaches
in a separate crate on a branch, and this will help.

/cc @maxdeviant @bennetbo @rtfeldman

Closes #ISSUE

Release Notes:

- N/A

crates/language_models/src/provider/copilot_chat.rs

@@ -368,6 +368,7 @@ pub fn map_to_language_model_completion_events(
                                                     id: tool_call.id.into(),
                                                     name: tool_call.name.as_str().into(),
                                                     is_input_complete: true,
+                                                    raw_input: tool_call.arguments.clone(),
                                                     input: serde_json::Value::from_str(
                                                         &tool_call.arguments,
                                                     )?,