language_models: Fix high memory consumption while using Agent Panel (#35764)

Closes #31108

The `num_tokens_from_messages` method we use from `tiktoken-rs` creates
new BPE every time that method is called. This creation of BPE is
expensive as well as has some underlying issue that keeps memory from
releasing once the method is finished, specifically noticeable on Linux.
This leads to a gradual increase in memory every time that method is
called in my case around +50MB on each call. We call this method with
debounce every time user types in Agent Panel to calculate tokens. This
can add up really fast.

This PR lands quick fix, while I/maintainers figure out underlying
issue. See upstream discussion:
https://github.com/zurawiki/tiktoken-rs/issues/39.

Here on fork https://github.com/zed-industries/tiktoken-rs/pull/1,
instead of creating BPE instances every time that method is called, we
use singleton BPE instances instead. So, whatever memory it is holding
on to, at least that is only once per model.

Before: Increase of 700MB+ on extensive use

On init:
<img width="500" alt="prev-init"
src="https://github.com/user-attachments/assets/70da7c44-60cb-477b-84aa-7dd579baa3da"
/>
First message:
<img width="500" alt="prev-first-call"
src="https://github.com/user-attachments/assets/599ffc48-3ad3-4729-b94c-6d88493afdbf"
/>
Extensive use:
<img width="500" alt="prev-extensive-use"
src="https://github.com/user-attachments/assets/e0e6b688-6412-486d-8b2e-7216c6b62470"
/>

After: Increase of 50MB+ on extensive use
On init:
<img width="500" alt="now-init"
src="https://github.com/user-attachments/assets/11a2cd9c-20b0-47ae-be02-07ff876e68ad"
/>
First message:
<img width="500" alt="now-first-call"
src="https://github.com/user-attachments/assets/ef505f8d-cd31-49cd-b6bb-7da3f0838fa7"
/>
Extensive use: 
<img width="500" alt="now-extensive-use"
src="https://github.com/user-attachments/assets/513cb85a-a00b-4f11-8666-69103a9eb2b8"
/>

Release Notes:

- Fixed issue where Agent Panel would cause high memory consumption over
prolonged use.

Cargo.toml

@@ -601,7 +601,7 @@ sysinfo = "0.31.0"
 take-until = "0.2.0"
 tempfile = "3.20.0"
 thiserror = "2.0.12"
-tiktoken-rs = "0.7.0"
+tiktoken-rs = { git = "https://github.com/zed-industries/tiktoken-rs", rev = "30c32a4522751699adeda0d5840c71c3b75ae73d" }
 time = { version = "0.3", features = [
     "macros",
     "parsing",