Move rustls provider setup to entrypoints

[yeoleobun]

Summary

• Move the rustls provider install out of ApplicationContext::new().
• Install the provider directly in executable entrypoints that can construct rustls-backed clients.
• Add an integration-test binary initializer so E2E tests install the provider before PBX library code runs.
• Keep provider setup fail-fast with expect(...).

Validation

• cargo check --bin rustpbx --bin sipflow --bin verify-i18n
• cargo test --test e2e_p2p_comprehensive_test --test e2e_p2p_demo_test --test e2e_queue_comprehensive_test --test e2e_conference_test -- --nocapture
• cargo test --features wholesale --test e2e_wholesale_test -- --nocapture
• Searched captured E2E logs for No rustls crypto provider, alsa-sys, failed to compile sipbot, panicked at, and background task panic; no matches.

Draft PR is for CI confirmation before merging.