Transfer: avoid checksum-path crash when JFR message source is unresolved

[Copilot]

In 11.2, GridFTP checksum flows (CKSM) can execute Transfer.readNameSpaceEntry* before a cell address is set, causing IllegalStateException from JFR message instrumentation and failing checksum negotiation. This regressed checksum handling for clients that request/verify checksums during upload.

• Problem focus

  • Transfer JFR event source construction assumed _cellAddress is always initialized.
  • Checksum-calculation transfer paths can legally run without that initialization.

• Code changes

  • Added a safe source builder in Transfer:
    • returns "<unknown>" when _cellAddress is absent
    • otherwise returns cell@domain
  • Replaced direct getCellName() + "@" + getDomainName() usage in JFR event setup for:
    • namespace read (PnfsGetFileAttributes)
    • pool selection
    • mover start
    • mover kill

• Regression coverage

  • Added TransferTest to verify namespace read works when no cell address is set (the failing precondition in this issue).

private synchronized String getMessageEventSource() {
    return _cellAddress == null ? "<unknown>"
          : _cellAddress.getCellName() + "@" + _cellAddress.getCellDomainName();
}