zeta2: Provider setup (#38676)

Creates a new `EditPredictionProvider` for zeta2, that requests
completions from a new cloud endpoint including context from the new
`edit_prediction_context` crate. This is not ready for use, but it
allows us to iterate.

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>
Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/edit_prediction_context/src/declaration.rs

@@ -41,6 +41,20 @@ impl Declaration {
         }
     }
 
+    pub fn parent(&self) -> Option<DeclarationId> {
+        match self {
+            Declaration::File { declaration, .. } => declaration.parent,
+            Declaration::Buffer { declaration, .. } => declaration.parent,
+        }
+    }
+
+    pub fn as_buffer(&self) -> Option<&BufferDeclaration> {
+        match self {
+            Declaration::File { .. } => None,
+            Declaration::Buffer { declaration, .. } => Some(declaration),
+        }
+    }
+
     pub fn project_entry_id(&self) -> ProjectEntryId {
         match self {
             Declaration::File {
@@ -52,6 +66,13 @@ impl Declaration {
         }
     }
 
+    pub fn item_range(&self) -> Range<usize> {
+        match self {
+            Declaration::File { declaration, .. } => declaration.item_range_in_file.clone(),
+            Declaration::Buffer { declaration, .. } => declaration.item_range.clone(),
+        }
+    }
+
     pub fn item_text(&self) -> (Cow<'_, str>, bool) {
         match self {
             Declaration::File { declaration, .. } => (
@@ -83,6 +104,16 @@ impl Declaration {
             ),
         }
     }
+
+    pub fn signature_range_in_item_text(&self) -> Range<usize> {
+        match self {
+            Declaration::File { declaration, .. } => declaration.signature_range_in_text.clone(),
+            Declaration::Buffer { declaration, .. } => {
+                declaration.signature_range.start - declaration.item_range.start
+                    ..declaration.signature_range.end - declaration.item_range.start
+            }
+        }
+    }
 }
 
 fn expand_range_to_line_boundaries_and_truncate(