feat(adk): retry A2A requests on transport error

[anthonyhaussman]

Problem

Same-cluster agent-to-agent (A2A) calls (pod-to-pod on port 8080) go directly through remoteA2ATool's HTTP client and never pass through any gateway/proxy layer.
When the target agent pod is evicted or replaced mid-request (e.g. node consolidation, rolling update), the in-flight connection is torn down and SendMessage fails with a raw transport error such as:

failed to send HTTP request: Post "http://<agent>.<ns>:8080": EOF

remoteA2AState's handleFirstCall / handleResume each call client.SendMessage exactly once.
There is no retry anywhere in the A2A client stack (remote_a2a_tool.go, a2aclient, or the underlying a2a-go transport), so any pod churn during a call surfaces directly to the end user as a hard failure, even though a retry a few hundred milliseconds later would very likely succeed once the Service's endpoint list catches up.

Fix

Add a small http.RoundTripper (retry_transport.go) that wraps the A2A tool's http.Client and retries only on transport-level failures:
io.EOF, io.ErrUnexpectedEOF, and *net.OpError (dial/read/write failures).

It never retries based on a received response status code, and it never retries when the request body cannot be safely replayed (req.GetBody == nil).

Retry policy: up to 3 attempts total, with a 250ms base delay that doubles between attempts, bounded by the request's context deadline.

The transport is wired in as the innermost layer, underneath the existing otelhttp transport, so a single logical A2A call (including any internal retries) still produces one trace span.

This covers both client.SendMessage call sites in remote_a2a_tool.go as well as agent-card resolution agentcard.NewResolver(...).Resolve), since both share the same wrapped http.Client.

Configuration

The retry is disabled by default and opt-in via env var, tunable through:

• KAGENT_A2A_RETRY_ENABLED (bool, default false)
• KAGENT_A2A_RETRY_MAX_ATTEMPTS (int, default 3)
• KAGENT_A2A_RETRY_BASE_DELAY (duration, default 250ms)