[Feat]: feat(transport): add stdio transport to run A2A protocol over process pipes

[muscariello]

Is your feature request related to a problem? Please describe.

Support a2a protocol over stdio in the Python SDK.

Describe the solution you'd like

A stdio transport for A2A lets the Python client speak the existing A2A protocol directly over a spawned process’s stdin/stdout instead of HTTP or gRPC, enabling fully local, port‑less, hermetic agent execution. The SDK would launch an agent (or model/runtime) with a --stdio flag, exchange line‑delimited JSON-RPC messages, and multiplex streaming task events through a single reader loop that dispatches responses and updates by request ID. This approach reduces overhead and dependencies (no network stack), improves security by narrowing exposure to a private child process, and unlocks embedding scenarios (CI, plugins, air‑gapped systems) while preserving the current ClientTransport interface, interceptors, telemetry, and error semantics.

Describe alternatives you've considered

No response

Additional context

No response

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