fix(a2a): stop a converter failure from stalling the A2A event stream

[copybara-service]

fix(a2a): stop a converter failure from stalling the A2A event stream

StreamHandler.handleEvent called ResponseConverter.clientEventToEvent with no
guard, and nothing between it and the transport's read loop catches a runtime
exception either: the a2a client's streaming dispatch catches only
A2AClientError, the SSE listener catches only JsonProcessingException, and
the JDK SSE subscriber runs subscription.request(1) only after the message
consumer returns. So a throw during conversion left the stream with no further
demand and no terminal signal - a hung Flowable rather than a failed one.

Route runtime exceptions to handleError, which is what a transport error
already uses, so the caller sees a failed stream instead of one that never ends.

Reachable today: a peer FilePart whose FileWithBytes.bytes is not valid
base64 makes PartConverter throw IllegalArgumentException. The new
off-thread test reproduces the stall without this fix - the subscriber times out
with no error and no completion - and both tests pass with it.