fix(core): preserve Field descriptions on variadic tool parameters - #4951

Summary

This pull request fixes lost Pydantic Field descriptions on variadic tool parameters.

*scores: Annotated[int, Field(description="Exam scores", ge=0, le=100)] advertises the constraints on each collected value but drops the description, so the model is shown an undocumented array argument. The same applies to values collected through **kwargs. Descriptions already reach variadic parameters through docstrings (#3956) and through an annotated string, leaving Field(description=...) as the one documented spelling that still loses them.

A Field() inside Annotated splits into attributes such as description and constraints collected as metadata. #4739 routed the metadata half to each collected value; this carries the description half to the parameter the schema advertises. It applies only when neither a docstring nor an annotated string supplies a description, so the existing precedence is unchanged, and reading only the description keeps the container's default_factory and the value-level constraint placement #4739 established. Extending the shared _extract_description_from_metadata helper would be broader than the need: it feeds every parameter kind and would also reorder precedence between a Field(description=...) and a sibling string for ordinary parameters.

Test plan

• test_variadic_field_description_in_schema covers *args and **kwargs under strict and non-strict schemas, asserting the description lands on the parameter while the constraints from the same Field(...) stay on each collected value. Both cases fail on main with KeyError: 'description'.
• Two precedence tests pin that a docstring and an annotated string still take priority.
• make format, make lint, make typecheck and uv run mypy --platform win32 src are clean.
• .agents/skills/code-change-verification/scripts/run.sh does not complete in my environment: 14 tests in tests/test_code_change_verification_runner.py fail with Timed out waiting for a controlled process transition. They reproduce identically on unmodified main here and are unrelated to this change. Excluding that file, the suite reports 9673 passed, 32 skipped.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR