Use u64 instead of usize in ElementId (#29493)

Truncation to a 32 bit `usize` could cause two distinct IDs to be
considered the same element.

Release Notes:

- N/A

crates/agent/src/context.rs

@@ -1,5 +1,4 @@
 use std::hash::{Hash, Hasher};
-use std::usize;
 use std::{ops::Range, path::Path, sync::Arc};
 
 use collections::HashSet;
@@ -85,7 +84,7 @@ impl AgentContext {
 /// ID created at time of context add, for use in ElementId. This is not the stable identity of a
 /// context, instead that's handled by the `PartialEq` and `Hash` impls of `AgentContextKey`.
 #[derive(Debug, Copy, Clone)]
-pub struct ContextId(usize);
+pub struct ContextId(u64);
 
 impl ContextId {
     pub fn zero() -> Self {
@@ -93,7 +92,7 @@ impl ContextId {
     }
 
     fn for_lookup() -> Self {
-        ContextId(usize::MAX)
+        ContextId(u64::MAX)
     }
 
     pub fn post_inc(&mut self) -> Self {

crates/agent/src/context_picker.rs

@@ -388,7 +388,7 @@ impl ContextPicker {
                 ContextMenuItem::custom_entry(
                     move |_window, cx| {
                         render_file_context_entry(
-                            ElementId::NamedInteger("ctx-recent".into(), ix),
+                            ElementId::named_usize("ctx-recent", ix),
                             worktree_id,
                             &path,
                             &path_prefix,

crates/agent/src/context_picker/file_context_picker.rs

@@ -169,7 +169,7 @@ impl PickerDelegate for FileContextPickerDelegate {
                 .inset(true)
                 .toggle_state(selected)
                 .child(render_file_context_entry(
-                    ElementId::NamedInteger("file-ctx-picker".into(), ix),
+                    ElementId::named_usize("file-ctx-picker", ix),
                     WorktreeId::from_usize(mat.worktree_id),
                     &mat.path,
                     &mat.path_prefix,

crates/agent/src/context_picker/symbol_context_picker.rs

@@ -171,10 +171,7 @@ impl PickerDelegate for SymbolContextPickerDelegate {
         let mat = &self.matches[ix];
 
         Some(ListItem::new(ix).inset(true).toggle_state(selected).child(
-            render_symbol_context_entry(
-                ElementId::NamedInteger("symbol-ctx-picker".into(), ix),
-                mat,
-            ),
+            render_symbol_context_entry(ElementId::named_usize("symbol-ctx-picker", ix), mat),
         ))
     }
 }