Fix stable totals for age-filtered clinical trial search

[imaurer]

Summary

• Age-filtered trial searches previously reported inconsistent totals: the count was derived from a prefix of the result set, so --count-only output changed with --limit
• Add count_all which traverses all CTGov pages (1000 results/page) and applies age verification across the complete population, returning the definitive verified count
• Route --count-only through count_all so count-only calls always return the true verified total (e.g. Total: 472 for melanoma/recruiting/age-51, stable across any --limit)
• Paginated search_page calls now return total=None when traversal does not exhaust all pages, surfacing "total unknown" in the CLI footer instead of a misleading lower bound
• Expose ClinicalTrialsClient::new_for_test under #[cfg(test)] for unit testing
• Add age-filter count stability spec assertions to spec/04-trial.md

Test plan

• cargo test passes (574 tests)
• make spec passes (96 passed, 5 skipped)
• biomcp search trial -c melanoma -s recruiting --age 51 --count-only returns Total: 472
• Same command with --limit 10, --limit 20, --limit 50 all return Total: 472
• biomcp search trial -c melanoma -s recruiting --age 51 --limit 10 shows total unknown footer
• biomcp search trial -c melanoma -s recruiting --age 51 --limit 10 --offset 20 shows paginated rows with total unknown