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/ollama/src/ollama.rs

@@ -35,18 +35,18 @@ impl Default for KeepAlive {
 pub struct Model {
     pub name: String,
     pub display_name: Option<String>,
-    pub max_tokens: usize,
+    pub max_tokens: u64,
     pub keep_alive: Option<KeepAlive>,
     pub supports_tools: Option<bool>,
     pub supports_vision: Option<bool>,
     pub supports_thinking: Option<bool>,
 }
 
-fn get_max_tokens(name: &str) -> usize {
+fn get_max_tokens(name: &str) -> u64 {
     /// Default context length for unknown models.
-    const DEFAULT_TOKENS: usize = 4096;
+    const DEFAULT_TOKENS: u64 = 4096;
     /// Magic number. Lets many Ollama models work with ~16GB of ram.
-    const MAXIMUM_TOKENS: usize = 16384;
+    const MAXIMUM_TOKENS: u64 = 16384;
 
     match name.split(':').next().unwrap() {
         "phi" | "tinyllama" | "granite-code" => 2048,
@@ -67,7 +67,7 @@ impl Model {
     pub fn new(
         name: &str,
         display_name: Option<&str>,
-        max_tokens: Option<usize>,
+        max_tokens: Option<u64>,
         supports_tools: Option<bool>,
         supports_vision: Option<bool>,
         supports_thinking: Option<bool>,
@@ -93,7 +93,7 @@ impl Model {
         self.display_name.as_ref().unwrap_or(&self.name)
     }
 
-    pub fn max_token_count(&self) -> usize {
+    pub fn max_token_count(&self) -> u64 {
         self.max_tokens
     }
 }
@@ -165,7 +165,7 @@ impl ChatRequest {
 // https://github.com/ollama/ollama/blob/main/docs/modelfile.md#valid-parameters-and-values
 #[derive(Serialize, Default, Debug)]
 pub struct ChatOptions {
-    pub num_ctx: Option<usize>,
+    pub num_ctx: Option<u64>,
     pub num_predict: Option<isize>,
     pub stop: Option<Vec<String>>,
     pub temperature: Option<f32>,
@@ -183,8 +183,8 @@ pub struct ChatResponseDelta {
     pub done_reason: Option<String>,
     #[allow(unused)]
     pub done: bool,
-    pub prompt_eval_count: Option<u32>,
-    pub eval_count: Option<u32>,
+    pub prompt_eval_count: Option<u64>,
+    pub eval_count: Option<u64>,
 }
 
 #[derive(Serialize, Deserialize)]