fix: shorten generated tool-call ids to 9 alphanumeric chars (fixes #1375)

[u7k4rs6]

Summary

• ToolCallFormatter._format in server.py falls back to str(uuid.uuid4()) when a tool call has no id field
• str(uuid.uuid4()) is 36 characters with hyphens; Mistral v3 chat templates enforce a 9-character alphanumeric constraint and raise TemplateError when this is violated
• This causes apply_chat_template to raise on the second turn of any multi-turn tool exchange with affected models, which the server surfaces as HTTP 404

Change

# before
tc_id = tc.pop("id", None) or str(uuid.uuid4())
# after
tc_id = tc.pop("id", None) or uuid.uuid4().hex[:9]

uuid.uuid4().hex[:9] yields 9 lowercase hex characters (e.g. 0819261ae) — valid under the Mistral v3 constraint and accepted by all other templates tested.

Affected models

mlx-community/Ministral-8B-Instruct-2410-4bit, Mistral-7B-Instruct-v0.3, Mixtral, Mistral-Nemo (any checkpoint sharing the v3 template family). Devstral-Small-2-...-2512 is unaffected as its template has no id-length constraint.

Closes #1375