Lint: fix real int-truncation bug; document intentional gosec findings

- TransportPool configKey: write conn-count ints verbatim (strconv) instead of
  byte() truncation — a config value >=256 previously truncated and made two
  distinct configs collide on one key, silently sharing a transport with the
  wrong settings (real fix, gosec G115).
- Document design-intentional gosec findings with //nolint:gosec + rationale:
  G117 (clients_secret intentionally marshaled into stored creds JSON),
  G704 (tokenURL is the trusted discovery endpoint), G124 (delete/test cookie,
  Secure set conditionally), G118 (long-lived background refresh) and pool
  utils short-ID rune, testutil mock server redirects.

Result: full-tree golangci-lint now reports 0 issues (was ~12). Build, vet,
and the full test suite remain green.

session.go

@@ -1007,7 +1007,7 @@ func (sm *SessionManager) CleanupOldCookies(w http.ResponseWriter, r *http.Reque
 						continue
 					}
 
-					deleteCookie := &http.Cookie{
+					deleteCookie := &http.Cookie{ //nolint:gosec // delete cookie sets Secure conditionally on the request proto
 						Name:     cookie.Name,
 						Value:    "",
 						Path:     "/",