test: re-read instead of trusting the partial-update write response

[mogita]

Problem

TestChatChannelIntegration/PartialUpdateChannel is the longest-running flake in this suite. It failed on main in the runs for #137 and #142, the unset half was later fixed, and it now fails on the set half instead:

chat_channel_integration_test.go:129
expected: string("red")
actual  : <nil>(<nil>)

The assertion reads Custom["color"] straight off the UpdateChannelPartial response, and the server can hydrate that response from a read replica that has not observed the write yet.

Solution

Assert the value by re-reading with require.Eventually, which is exactly what the unset half of the same test already does, and for the same stated reason. Both halves are now symmetric and neither depends on read-after-write.

No coverage is lost. The value assertion still catches a Set map serialized under the wrong key or a Custom map that fails to deserialize, which is the bug this test exists for, and the write response and GetOrCreate deserialize the same ChannelResponse type. Only the assumption that the write response is immediately consistent is gone.

How to verify

1. go test -run 'TestChatChannelIntegration/PartialUpdateChannel' . passes. I ran it 10 times in a row against the integration app: 10 pass, 0 fail.
2. A real regression still fails: break the Set key or the Custom deserialization and the Eventually never satisfies, so the test fails after 5s with color should be set to red.