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/language_models/src/provider/deepseek.rs

@@ -49,8 +49,8 @@ pub struct DeepSeekSettings {
 pub struct AvailableModel {
     pub name: String,
     pub display_name: Option<String>,
-    pub max_tokens: usize,
-    pub max_output_tokens: Option<u32>,
+    pub max_tokens: u64,
+    pub max_output_tokens: Option<u64>,
 }
 
 pub struct DeepSeekLanguageModelProvider {
@@ -306,11 +306,11 @@ impl LanguageModel for DeepSeekLanguageModel {
         format!("deepseek/{}", self.model.id())
     }
 
-    fn max_token_count(&self) -> usize {
+    fn max_token_count(&self) -> u64 {
         self.model.max_token_count()
     }
 
-    fn max_output_tokens(&self) -> Option<u32> {
+    fn max_output_tokens(&self) -> Option<u64> {
         self.model.max_output_tokens()
     }
 
@@ -318,7 +318,7 @@ impl LanguageModel for DeepSeekLanguageModel {
         &self,
         request: LanguageModelRequest,
         cx: &App,
-    ) -> BoxFuture<'static, Result<usize>> {
+    ) -> BoxFuture<'static, Result<u64>> {
         cx.background_spawn(async move {
             let messages = request
                 .messages
@@ -335,7 +335,7 @@ impl LanguageModel for DeepSeekLanguageModel {
                 })
                 .collect::<Vec<_>>();
 
-            tiktoken_rs::num_tokens_from_messages("gpt-4", &messages)
+            tiktoken_rs::num_tokens_from_messages("gpt-4", &messages).map(|tokens| tokens as u64)
         })
         .boxed()
     }
@@ -365,7 +365,7 @@ impl LanguageModel for DeepSeekLanguageModel {
 pub fn into_deepseek(
     request: LanguageModelRequest,
     model: &deepseek::Model,
-    max_output_tokens: Option<u32>,
+    max_output_tokens: Option<u64>,
 ) -> deepseek::Request {
     let is_reasoner = *model == deepseek::Model::Reasoner;