Bump common.ai floor to pydantic-ai-slim>=1.71.0 and document capabilities passthrough (#67444)

1.71.0 is the first release that ships the `pydantic_ai.capabilities`
module (Thinking, WebSearch, ImageGeneration, MCP, etc.). Bumping the
floor lets users pass capabilities through `AgentOperator(agent_params=...)`
without operator-level support landing first.

The new example DAG demonstrates the pattern with Thinking, WebSearch,
and composition with SQLToolset. The agent.rst guide adds a short
"Capabilities (pydantic-ai)" section with a warning about the templated-
field serialization gap -- first-class `capabilities=` support is left
as a follow-up.

Generated metadata (README.rst, docs/index.rst, uv.lock) updated to match.