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.

refresh_coordinator.go

@@ -229,7 +229,7 @@ func (rc *RefreshCoordinator) CoordinateRefresh(
 
 	if isNew {
 		// We created a new operation, so we need to execute it
-		go rc.executeRefreshAsync(operation, sessionID, tokenHash, refreshFunc)
+		go rc.executeRefreshAsync(operation, sessionID, tokenHash, refreshFunc) //nolint:gosec // long-lived background refresh intentionally uses a background context
 	} else {
 		// Joined existing operation - this is a deduplicated request
 		atomic.AddInt64(&rc.metrics.deduplicatedRequests, 1)