Python: pass AG-UI state through AgentSession

[Bortlesboat]

Motivation and Context

Fixes #5197.

AG-UI already carried request state in metadata and model-facing state injection, but it never copied that state into AgentSession.state. That left ContextProvider implementations with an empty session state when runs came through the AG-UI path.

Description

• copy AG-UI request state into the created AgentSession
• add a regression test that captures the session passed to agent.run(...) and asserts the request state is available there

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add [BREAKING] prefix to the title of the PR.