fix(test): align GetExecutionByID twin test with (nil, nil) contract

Sibling-consistency miss from 1017f66 (which aligned the test in
internal/config/store_postgres_db_test.go to the contract that
GetExecutionByID returns (nil, nil) for missing rows): the duplicate
"Get execution by ID - not found" subtest in store_postgres_test.go
was left on the pre-contract `assert.Error + err.Error()` shape, so
the Integration Tests job panicked with a nil-pointer deref the moment
it tried to format the nil err. The panic cascaded through
TestPostgresStore_PurchaseExecutions and aborted the rest of the
internal/config suite -- visible in the PR's failing CI as
`SIGSEGV ... store_postgres_test.go:313`.

Bring this twin test in line with the contract documented in
1017f66 and the sibling subtest at store_postgres_db_test.go:594:
expect (nil, nil), assert the returned execution is nil, no err
deref. GetExecutionByPlanAndDate's not-found assertion below stays
as-is -- that one DOES wrap a "not found" error (store_postgres.go:1317).

Author: cristim

internal/config/store_postgres_test.go

@@ -307,10 +307,15 @@ func TestPostgresStore_PurchaseExecutions(t *testing.T) {
 	})
 
 	t.Run("Get execution by ID - not found", func(t *testing.T) {
-		// Use a valid UUID format that doesn't exist
-		_, err := store.GetExecutionByID(ctx, "00000000-0000-0000-0000-000000000000")
-		assert.Error(t, err)
-		assert.Contains(t, err.Error(), "not found")
+		// Use a valid UUID format that doesn't exist.
+		// Contract: GetExecutionByID returns (nil, nil) on not-found; every
+		// caller maps the nil execution to its own not-found error. The
+		// sibling assertion in store_postgres_db_test.go was aligned to this
+		// contract in 1017f66a8; this twin test panicked with a nil-pointer
+		// deref on err.Error() until brought in line here.
+		exec, err := store.GetExecutionByID(ctx, "00000000-0000-0000-0000-000000000000")
+		require.NoError(t, err)
+		assert.Nil(t, exec)
 	})
 
 	t.Run("Get execution by plan and date - not found", func(t *testing.T) {