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.

internal/testutil/suite.go

@@ -72,7 +72,7 @@ func (s *OIDCSuite) NewRequest(method, path string) *http.Request {
 // NewRequestWithCookie creates a request with a session cookie
 func (s *OIDCSuite) NewRequestWithCookie(method, path, cookieName, cookieValue string) *http.Request {
 	req := s.NewRequest(method, path)
-	req.AddCookie(&http.Cookie{
+	req.AddCookie(&http.Cookie{ //nolint:gosec // test cookie on an httptest request
 		Name:  cookieName,
 		Value: cookieValue,
 	})