Enable TCPing for single-server custom profiles

[eliotcougar]

Summary

TCPing currently skips every custom profile. Resolve its target from the saved raw JSON so configs with exactly one valid server address and port can be tested. Multiple-server configs and invalid endpoints remain skipped, and stale cached addresses are never used as a fallback.

Depends on #6235. Until that parser fix is merged, this PR also includes the parser changes. The TCPing-only change is limited to the worker and its JVM regression tests.

Related to #6233 (custom-profile TCPing).

Validation

• 107 JVM tests passed, including 8 new TCPing endpoint cases.
• Play Store debug and release builds passed; native packaging checked for all ABIs.
• Emulator UI tests confirmed successful TCP connections for two single-server custom configs and a normal control, with no connections to multiple-server or stale cached endpoints. Stored results and raw JSON were checked afterward.

Not run: physical-device, public-server connectivity, or native real-delay runtime tests. Emulator validation used a local TCP listener with the app's declared local-network permission granted.