Standardize on u64 for token counts (#32869)

Previously we were using a mix of `u32` and `usize`, e.g. `max_tokens:
usize, max_output_tokens: Option<u32>` in the same `struct`.

Although [tiktoken](https://github.com/openai/tiktoken) uses `usize`,
token counts should be consistent across targets (e.g. the same model
doesn't suddenly get a smaller context window if you're compiling for
wasm32), and these token counts could end up getting serialized using a
binary protocol, so `usize` is not the right choice for token counts.

I chose to standardize on `u64` over `u32` because we don't store many
of them (so the extra size should be insignificant) and future models
may exceed `u32::MAX` tokens.

Release Notes:

- N/A

crates/assistant_context_editor/src/context_editor.rs

@@ -3121,12 +3121,12 @@ fn invoked_slash_command_fold_placeholder(
 
 enum TokenState {
     NoTokensLeft {
-        max_token_count: usize,
-        token_count: usize,
+        max_token_count: u64,
+        token_count: u64,
     },
     HasMoreTokens {
-        max_token_count: usize,
-        token_count: usize,
+        max_token_count: u64,
+        token_count: u64,
         over_warn_threshold: bool,
     },
 }
@@ -3139,9 +3139,7 @@ fn token_state(context: &Entity<AssistantContext>, cx: &App) -> Option<TokenStat
         .model;
     let token_count = context.read(cx).token_count()?;
     let max_token_count = model.max_token_count();
-
-    let remaining_tokens = max_token_count as isize - token_count as isize;
-    let token_state = if remaining_tokens <= 0 {
+    let token_state = if max_token_count.saturating_sub(token_count) == 0 {
         TokenState::NoTokensLeft {
             max_token_count,
             token_count,
@@ -3182,7 +3180,7 @@ fn size_for_image(data: &RenderImage, max_size: Size<Pixels>) -> Size<Pixels> {
     }
 }
 
-pub fn humanize_token_count(count: usize) -> String {
+pub fn humanize_token_count(count: u64) -> String {
     match count {
         0..=999 => count.to_string(),
         1000..=9999 => {