Zalo adapter — webhook-signature test-path review (closes Echo's flag)

Closes open item 6.6b/§9.5 of eve-customer-service-design: "the security review of this adapter should also check whether v0.1.0 has any tests for its webhook-signature path." Verdict: ✅ the path exists and is tested — 3 dedicated tests, 106/106 pass, 100% branch coverage. Echo's flag cleared with two residual notes (§5).