feat(app-server): accept Codex Desktop startup calls

[shiny-code-bot]

Summary

• accept Codex Desktop's app-server --analytics-default-enabled launch flag as a no-op in both app-server entrypoints
• preserve top-level code app-server --listen support while adding the desktop flag
• return harmless initialized responses for Codex Desktop startup polling methods instead of Invalid request
• add a binary smoke covering the desktop launch flag and baseline app-server polling methods

Refs #377

Validation

• cargo test -p code-app-server binary_smoke_accepts_desktop_startup_polling_methods -- --nocapture
• ./build-fast.sh
• built binary parse smoke: code app-server --analytics-default-enabled --listen stdio:// --help
• Claude Sonnet review found no blocking correctness issues; actionable nits for --listen and list response shape were fixed before PR

Notes

• This is the first desktop-local compatibility slice. It stops the known launch flag failure and baseline invalid-request polling loop, but it intentionally returns empty/no-op data for unsupported desktop surfaces such as plugins, hooks, remote control, and experimental feature enablement.