false report of epoch drop in tx_st40p

[DianaEs2]

When tx_session starts after mtl_start, stat_epoch_drop is shown as a huge number.

Tested this commit f91c025 ("Add: St40p support in RxTxApp", 2025-12-11)

With this config local_tx_1d.json

With "--runtime_session" argument:
"program": "${workspaceFolder}/tests/tools/RxTxApp/build/RxTxApp",
"args": ["--config_file", "${workspaceFolder}/config/local_tx_1d.json", "--log_level", "info", "--runtime_session"],

The MTL log is captured here mtl_out.log

Find this line in the log:
MTL: 2025-12-23 09:22:18, TX_ANC_SESSION(0): epoch drop 2804886818

It happens because tx_ancillary_session_init_pacing_epoch is not called with --runtime_session argument.