fix: replace dispatch-core with OkHttp3 in test infrastructure #1026

Job	Run time
compile	5m 41s
test (7, model + views + customer + util + small data + berlin + v2_x, code.model code.views code...	2m 23s
test (6, v7 + http4sbridge + UKOpenBanking, code.api.v7_0_0 code.api.http4sbridge code.api.UKOpen...	2m 47s
test (5, ResourceDocs + v3_1 + v1_4 + v1_3, code.api.ResourceDocs1_4_0 code.api.v3_1_0 code.api.v...	3m 14s
test (4, v5_1 + v5_0 + v3_0, code.api.v5_1_0 code.api.v5_0_0 code.api.v3_0_0)	2m 59s
test (9, v4 Dynamic tests, code.api.v4_0_0.Dynamic)	3m 16s
test (8, connector + auth + login + mgmt + metrics + remaining (catch-all), code.connector code.u...	2m 0s
test (3, v6 only, code.api.v6_0_0)	2m 29s
test (2, v1_2_1 only (largest unsplittable suite, isolated), code.api.v1_2_1)	3m 41s
test (1, v4 non-Dynamic, code.api.v4_0_0.AccountAccessTest code.api.v4_0_0.AccountBalanceTest cod...	3m 7s
report	8s
	31m 45s

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