Share external DTD dictionary retry loader across DataDictionaryTest and MessageTest

[Copilot]

DataDictionaryTest.shouldLoadDictionaryWhenExternalDTDisEnabled could fail on external DTD fetch timeouts, while MessageTest already had a retry-based mitigation for the same failure mode. This change applies the same timeout/retry behavior to DataDictionaryTest and centralizes the logic so both tests use one implementation.

• Shared external DTD loader

  • Added ExternalDtdDataDictionaryLoader in quickfixj-base to encapsulate:
    • temporary connect/read timeout overrides
    • bounded retries with backoff
    • property restoration on exit

• DataDictionaryTest

  • Replaced the direct DataDictionary("FIX_External_DTD.xml", ...) construction with the shared loader so the test tolerates transient network timeouts when resolving the external DTD.

• MessageTest

  • Removed the test-local retry implementation and switched to the shared loader, keeping behavior aligned with DataDictionaryTest and avoiding duplicate logic.

DataDictionary dataDictionary =
        ExternalDtdDataDictionaryLoader.load("FIX_External_DTD.xml");