[Feat]: Add RetryTransport for automatic retry with exponential backoff

查看现有的传输类（JsonRpcTransport、RestTransport、GrpcTransport）以及 TenantTransportDecorator 模式。新的 RetryTransport 必须能够包装任意 ClientTransport。首先检查 SDK 的异常层次结构（A2AClientError、A2AClientTimeoutError），以及它们如何链接 httpx 和 grpc 错误。为 retry predicate 和 on_retry hook 编写测试。确保该实现适用于全部三种传输，并且不会重试领域错误。

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

The SDK's transports (JsonRpcTransport, RestTransport, GrpcTransport) raise immediately on transient failures (network errors, timeouts, rate limits, 5xx responses, gRPC UNAVAILABLE/RESOURCE_EXHAUSTED) with no built-in retry mechanism. Every caller has to reimplement the same retry/backoff loop, and to do it correctly they have to inspect __cause__ chains because A2AClientError doesn't expose HTTP status codes or gRPC codes directly.

Describe the solution you'd like

A RetryTransport decorator wrapping any ClientTransport, mirroring the existing TenantTransportDecorator pattern:

inner = JsonRpcTransport(httpx_client=client, agent_card=card)
transport = RetryTransport(base=inner, max_retries=3)

The default predicate retries on:

• A2AClientTimeoutError (always).
• A2AClientError chained from httpx.RequestError or httpx.HTTPStatusError(408/429/502/503/504).
• A2AClientError chained from grpc.aio.AioRpcError(UNAVAILABLE/RESOURCE_EXHAUSTED).
• Domain errors (TaskNotFoundError, etc.) are never retried.

retry_predicate is configurable for custom logic, and on_retry is exposed as a hook for logging/metrics. Streaming methods (send_message_streaming, subscribe) only retry before the first event is yielded.

Describe alternatives you've considered

• Retry via ClientCallInterceptor — not feasible; after() only fires on successful results, so interceptors never see exceptions.
• Transport-specific retry (httpx custom transport, gRPC retryPolicy / UnaryUnaryClientInterceptor) — operates below the SDK's exception layer, requires two separate configurations, and doesn't cover all three transports uniformly.
• Retry inside each transport implementation — triples the logic across JsonRpcTransport, RestTransport, GrpcTransport and forces every future transport to reimplement it.

Additional context

Purely additive: new RetryTransport class, tests, and one export change. No new dependencies — the grpc import is conditional, matching the SDK's existing pattern. ClientFactory integration is left as a follow-up.

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