Make error selector registry pluggable (avoid hard dependency on OpenChain)

[findolor]

Problem

Error decoding currently depends on OpenChain’s signature database, which makes results
nondeterministic as new selectors are registered over time.

Current behavior

selector_registry_abi_decode calls the OpenChain API, so selectors that used to
be unknown (for example 0xdeadbeef) can later become known and break tests that expect Unknown.

Impact

Flaky or brittle tests and an implicit network dependency in downstream repos.