language_models: Update tiktoken-rs to support newer models (#30951)

I was able to get this fix in upstream, so now we can have simpler code
paths for our model selection.

I also added a test to catch if this would cause a bug again in the
future.

Release Notes:

- N/A

Cargo.toml

@@ -555,7 +555,7 @@ sysinfo = "0.31.0"
 take-until = "0.2.0"
 tempfile = "3.20.0"
 thiserror = "2.0.12"
-tiktoken-rs = "0.6.0"
+tiktoken-rs = "0.7.0"
 time = { version = "0.3", features = [
     "macros",
     "parsing",