fix: mock signature retrieval (#671) #1603

Job	Run time
test (cli, redis_tests)	5m 5s
distributions (ubuntu-latest, linux, x86_64-unknown-linux-gnu, x64, glibc)	1m 37s
distributions (windows-latest, windows, x86_64-pc-windows-msvc, x64)	8m 45s
test (sdk, stacks-signers)	3m 20s
distributions (macos-latest, darwin, x86_64-apple-darwin, x64)	1m 34s
distributions (macos-latest, darwin, aarch64-apple-darwin, arm64)	1m 49s
semantic-release	30s
build-publish (hirosystems/chainhook, chainhook-linux-x64-glibc, dockerfiles/components/chainhook...	13m 29s
deploy-dev (mainnet)	1s
deploy-dev (testnet)	1s
auto-approve-dev	0s
auto-approve-stg	1s
deploy-staging	1s
deploy-prod	1s
	36m 14s

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