ingest: TestBSBProducerFnCallbackError is flaky, transient/race failures

[sreuland]

What version are you using?

mainline

What did you do?

go test -v ./...

What did you expect to see?

pass

What did you see instead?

Intermittently on GH CI runs, TestBSBProducerFnCallbackError fails with:

--- FAIL: TestBSBProducerFnCallbackError (0.06s)
    producer_test.go:244: FAIL:	GetFile(string,string)
        		at: [/home/runner/work/go-stellar-sdk/go-stellar-sdk/ingest/producer_test.go:240 /home/runner/work/go-stellar-sdk/go-stellar-sdk/ingest/producer_test.go:206]
    producer_test.go:244: FAIL: 3 out of 4 expectation(s) were met.
        	The code you are testing needs to make 1 more call(s).
        	at: [/home/runner/work/go-stellar-sdk/go-stellar-sdk/ingest/producer_test.go:244 /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1308 /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1572 /opt/hostedtoolcache/go/1.25.4/x64/src/testing/testing.go:1928]