feat: exercise every A2A RPC in the echo agent example

[janosSarusiKis]

Extends the echo agent to cover the A2A calls that were compiled but never
exercised at runtime, and fixes a server crash found while doing so.

Changes

• ClientMain runs one demo per call shape — SendMessage,
  SendStreamingMessage + GetTask, SubscribeToTask + CancelTask,
  GetExtendedAgentCard — selectable with --demo message,stream,cancel,card.
• EchoAgentExecutor picks its behaviour from a prefix on the message text:
  stream: <text> runs a task emitting one artifact per word, slow: <seconds>
  runs a cancellable long task, anything else replies with a plain message.
• A2AEventStream (new) decodes the raw SlimRPC response stream into A2A
  domain events.

Fix: protobuf runtime pin

proto-google-common-protos 2.70.0 is generated against protobuf 4.33.2 while
the runtime was pinned to 4.33.1. Building an AgentCard proto loads
com.google.api.AnnotationsProto, whose version check fails with an
ExceptionInInitializerError; SlimA2AHandler catches only A2AError, so that
Error escaped and killed the server process. This is the
GetExtendedAgentCard "hang" the example previously warned about — clients were
seeing DEADLINE_EXCEEDED against a dead server. Raising the pin to 4.33.2
fixes it, and the call now returns the card.

Verification

All four demos pass against a local SLIM 2.0.0 gateway, across three
consecutive client runs on one long-lived server. CI has no gateway, so it only
compiles these examples — the runtime results are from local runs.

Follow-ups (not in this PR)

• A2AEventStream belongs in slim-a2a: every caller of
  sendStreamingMessage/subscribeToTask needs it, since those return a
  ResponseStreamReader of protobuf bytes.
• SlimA2AHandler should contain RuntimeException/Error from a handler
  rather than letting it take down the agent.

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