fix(structured-output): strip examples from strict JSON schemas

[sylvesterkaczmarek]

Summary

• remove the JSON Schema examples metadata keyword when converting Pydantic models to the API's strict structured-output schema dialect
• strip the keyword recursively, including nested definitions
• preserve validation keywords and descriptive metadata
• add regression coverage and run the existing Pydantic schema tests

Fixes #2274

Test plan

• uv run ruff format src/openai/lib/_pydantic.py tests/lib/test_pydantic_examples.py
• uv run ruff check --fix tests/lib/test_pydantic_examples.py
• uv run ruff check src/openai/lib/_pydantic.py tests/lib/test_pydantic_examples.py
• uv run pytest -o addopts= -q tests/lib/test_pydantic_examples.py
• uv run pytest -o addopts= -q tests/lib/test_pydantic.py
• git diff --check

All focused verification steps passed before the branch was reduced to its single final commit.