fix(infer): validate explicit tool choices

[RerankerGuo]

PR type

• Bug Fix
• New Feature
• Document Updates
• More Models or Datasets Support

PR information

An explicit tool_choice can name a function that is absent from the
request's tools list. ChatCompletionRequestMixin intended to report this
as a ValueError, but next() raised StopIteration before the validation
branch could run.

This change gives next() a None default so the existing, actionable error
is raised:

ValueError: Tool choice 'missing' not found in tools.

Valid explicit choices retain the existing behavior of narrowing tools to
the selected function. Automatic and disabled tool selection are unchanged.

Experiment results

Before:

StopIteration: <empty message>

After:

ValueError: Tool choice 'missing' not found in tools.

Verification:

.venv/bin/python tests/run.py --test_dir tests/infer --pattern test_protocol.py
SUCCESS (Runs=2, success=2)

.venv/bin/pre-commit run --files swift/infer_engine/protocol.py tests/infer/test_protocol.py
All hooks passed