Return completion proposals from inline completion providers (#17578)

Updates the inline completion provider to return a completion proposal
which is then converted to a completion state. This completion proposal
includes more detailed information about which inlays specifically
should be rendered.

Release Notes:

- Added support for fill-in-the-middle style inline completions


![image](https://github.com/user-attachments/assets/1830700f-5a76-4d1f-ac6d-246cc69b64c5)

crates/copilot/src/copilot_completion_provider.rs

@@ -1,14 +1,14 @@
 use crate::{Completion, Copilot};
 use anyhow::Result;
 use client::telemetry::Telemetry;
-use editor::{Direction, InlineCompletionProvider};
+use editor::{CompletionProposal, Direction, InlayProposal, InlineCompletionProvider};
 use gpui::{AppContext, EntityId, Model, ModelContext, Task};
 use language::{
     language_settings::{all_language_settings, AllLanguageSettings},
     Buffer, OffsetRangeExt, ToOffset,
 };
 use settings::Settings;
-use std::{ops::Range, path::Path, sync::Arc, time::Duration};
+use std::{path::Path, sync::Arc, time::Duration};
 
 pub const COPILOT_DEBOUNCE_TIMEOUT: Duration = Duration::from_millis(75);
 
@@ -237,7 +237,7 @@ impl InlineCompletionProvider for CopilotCompletionProvider {
         buffer: &Model<Buffer>,
         cursor_position: language::Anchor,
         cx: &'a AppContext,
-    ) -> Option<(&'a str, Option<Range<language::Anchor>>)> {
+    ) -> Option<CompletionProposal> {
         let buffer_id = buffer.entity_id();
         let buffer = buffer.read(cx);
         let completion = self.active_completion()?;
@@ -267,7 +267,14 @@ impl InlineCompletionProvider for CopilotCompletionProvider {
             if completion_text.trim().is_empty() {
                 None
             } else {
-                Some((completion_text, None))
+                Some(CompletionProposal {
+                    inlays: vec![InlayProposal::Suggestion(
+                        cursor_position.bias_right(buffer),
+                        completion_text.into(),
+                    )],
+                    text: completion_text.into(),
+                    delete_range: None,
+                })
             }
         } else {
             None