Add Pydantic AI integration example

[sjsjsjjs534]

Summary

• Add a runnable Pydantic AI + Opik integration example using Logfire/OpenTelemetry instrumentation
• Include a dry-run fallback when Opik or provider credentials are missing
• Update the integrations index and root README table

Test Plan

• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples uv run pydantic-ai-opik --dry-run
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples uv run ruff check .
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples uv run ruff format --check .
• UV_CACHE_DIR=/private/tmp/uv-cache-opik-examples bash run.sh

Closes #22