[codex-analytics] enable general analytics by default

[rhan-oai]

Summary

• Make GeneralAnalytics stable and enabled by default.
• Update feature tests and app-server lifecycle fixtures for explicit general_analytics=false.
• Keep app-server integration tests isolated from host managed config so explicit feature fixtures are deterministic.

Validation

• cargo test -p codex-features
• cargo test -p codex-app-server general_analytics (matched 0 tests)
• cargo test -p codex-app-server thread_start_
• cargo test -p codex-app-server thread_fork_
• cargo test -p codex-app-server thread_resume_
• cargo test -p codex-app-server config_read_includes_system_layer_and_overrides

[owenlin0]

preapproving