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

### 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_

### Code of Conduct

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