Signal approximate totals for age-only trial count searches

[imaurer]

Summary

• Age-only --count-only trial searches now signal that the returned total is approximate, since BioMCP's age post-filter runs during full search rather than the fast count path
• Text output appends (approximate, age post-filtered) to the total; JSON output adds "approximate": true
• The --age help text notes the approximation behavior for CTGov-only fast-count paths
• COUNT_TRAVERSAL_PAGE_CAP is hoisted to the module-level constant block alongside other traversal constants

Details

When a user runs biomcp search trial --age N --count-only, BioMCP previously returned CTGov's upstream total without indicating that the actual result set may be smaller after client-side age filtering. This change makes that approximation explicit.

A new TrialCount enum (Exact, Approximate, Unknown) annotates the return type of count_all_with_ctgov_client and count_all. The Approximate variant fires only on the CTGov age-only fast path. All other count paths continue returning Exact or Unknown as before, with no change to their output.

Test plan

• make build passes
• make test passes (632 tests)
• make check passes (clippy + fmt)
• make spec passes (113 passed, 5 skipped)
• Age-only --count-only text output shows (approximate, age post-filtered) qualifier
• Age-only --count-only JSON output includes "approximate": true
• Non-age --count-only output is unchanged (no qualifier, no approximate field)