Use the singular in the --all-tenants summary line #135

With --all-tenants, the summary line on standard error always uses the plural:

$ hodeishield --all-tenants vendors list
...
1 vendors across 2 client tenants.

Expected: 1 vendor across 2 client tenants. (and across 1 client tenant when the portfolio has one readable client). The same applies to the other resources (alert, risk, evidence item, endpoint, control).

Where: crates/hodeishield-cli/src/commands/resources.rs, the --all-tenants summary ({total} {noun} across {n} client tenants.), added in #134. Standard error only; standard output and exit codes do not change. Add a test for the singular forms.