chore: add nextest cover profile, update backward-compat imports

Nextest cover profile for cargo llvm-cov integration.
Update existing test imports from load_workflow_from_str to
load_single_workflow_from_str for backward compatibility.

.config/nextest.toml

@@ -34,6 +34,17 @@ retries = 2
 [profile.ci.junit]
 path = "target/nextest/ci/junit.xml"
 
+[profile.cover]
+# Coverage profile — used with cargo llvm-cov nextest
+fail-fast = false
+test-threads = "num-cpus"
+failure-output = "immediate-final"
+success-output = "never"
+slow-timeout = { period = "60s", terminate-after = 2 }
+
+[profile.cover.junit]
+path = "target/nextest/cover/junit.xml"
+
 # Postgres tests must run serially (shared database state)
 [[profile.default.overrides]]
 filter = "package(wfe-postgres)"