feat(examples): add PydanticAI agent example + E2E test

[Rushit]

Goal

Add a PydanticAI agent example with E2E span validation against zradar.

Skeleton

examples/pydantic-ai/ on docs_opt:

examples/pydantic-ai/
  agent.py              ← Agent with lookup_fact tool, TestModel mock
  pyproject.toml        ← uv-managed, Python 3.11+
  README.md
  tests/expected_spans.json

What "done" looks like

• just example pydantic-ai runs without error (uses TestModel — no API key)
• just example-test pydantic-ai validates spans in zradar
• Works with OPENAI_API_KEY set

How to test

just dev
just example-test pydantic-ai

[Rushit]

Path update: Example is now at examples/pydantic-ai/python/example.py.