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.

dynamic_client_registration.go

@@ -374,7 +374,7 @@ func (r *DynamicClientRegistrar) saveCredentialsToStore(ctx context.Context, res
 func (r *DynamicClientRegistrar) saveCredentials(resp *ClientRegistrationResponse) error {
 	filePath := r.credentialsFilePath()
 
-	data, err := json.MarshalIndent(resp, "", "  ")
+	data, err := json.MarshalIndent(resp, "", "  ") //nolint:gosec // registration response incl. client_secret is the API output
 	if err != nil {
 		return fmt.Errorf("failed to marshal credentials: %w", err)
 	}