Improve tool schema diagnostic errors

[tarunag10]

Summary

• raise a clear ValueError when strict schema $ref resolution points at a missing path instead of leaking KeyError
• raise an actionable UserError when function tool parameters use Pydantic protected method names such as model_validate or model_dump
• add regression coverage for both diagnostics

Fixes #3551.
Fixes #3549.

Validation

• uv run pytest tests/test_strict_schema.py tests/test_function_schema.py -q
• uv run ruff format --check src/agents/strict_schema.py src/agents/function_schema.py tests/test_strict_schema.py tests/test_function_schema.py
• uv run ruff check src/agents/strict_schema.py src/agents/function_schema.py tests/test_strict_schema.py tests/test_function_schema.py