[fix] prove BFRT initialization in model CI

[ZER0-Nu1L]

What changed

• keep bfshell stdin open until the embedded BFRT command can run
• bootstrap the site-neutral profile inside the bf_switchd Python namespace, preserving the injected bfrt object
• write and verify a post-complete_operations() marker plus expected initialization output

Validation

• PYTHONPATH=... python3 -m unittest discover -s targets/tofino/runtime/tests -v
• GitHub Actions run 33089138514: compile and privileged Tofino model jobs passed
• model artifact contains bfrt-initialize.marker and profile/mapping output

The workflow still validates only the site-neutral Tofino 1 model, not physical hardware or site mappings.