fix(test): Stop ExampleEvent hanging when ticks beat registration

[Jrc356]

ExampleEvent hangs until the 10m test timeout on Windows CI. It hit main in run 32182612530 (both Windows jobs), and #78 in run 35921918139.

• Cause: the handler flushes the response headers before it registers the subscriber. Subscribe can therefore return, and the publisher can send all five 1ms ticks, before the subscriber is registered. Every tick then reaches nobody, and the example waits on stream.Events forever.
• Fix: register a Repository that replays nothing and set ReplayAll. The Server calls Replay only after it adds the subscriber, so the example starts publishing only after that call.
• Proof: in a scratch copy I added a 50ms sleep between the header flush and registration. The old example hangs until the test timeout; the new one passes in 0.4s.

ExampleRepository publishes right after Subscribe in the same way. It hasn't failed in CI, and this PR leaves it alone.

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