fix(api): generate Mistral-compliant tool-call ids

[sam-fakhreddine]

Fixes #1995.

Problem

omlx generates tool-call ids as call_<8 hex> (e.g. call_49c89af0). Mistral /
tekken (MistralCommonBackend) validates tool_call_id against [a-zA-Z0-9]
with a length of exactly 9 and rejects anything else when the assistant turn
is replayed on the next request:

HTTP 500: Tool call id was call_49c89af0 but must be a-z, A-Z, 0-9,
with a length of 9

So the first turn works and the second 500s — on the id the server itself
generated. Every multi-turn tool loop with Devstral / Mistral-Small breaks.

Fix

Centralize id creation in shared_models.generate_tool_call_id():

return f"call{uuid.uuid4().hex[:5]}"   # "call" + 5 hex = exactly 9 alphanumeric

Valid for Mistral and still an opaque, recognizable id for OpenAI / Anthropic
clients (which treat the id as opaque — nothing in the codebase matches on the
call_ prefix). Replaces all 17 inline f"call_{uuid.uuid4().hex[:8]}"
sites across tool_calling.py (10), server.py (3), anthropic_utils.py (2)
and responses_utils.py (2), and drops the now-unused uuid import where it was
only used for ids.

Tests

tests/test_shared_models.py::TestGenerateToolCallId — asserts the id is exactly
9 chars, matches [a-zA-Z0-9], has no underscore, and is unique across many
draws.