Expose Code Bridge status through app-server

[shiny-code-bot]

Summary

• Add experimental codeBridge/status/read app-server method for read-only Code Bridge availability/status.
• Add shared Code Bridge service status DTO plus a client GET /status helper.
• Keep app-server out of the bridge telemetry/control plane: no hello, subscribe, event replay, screenshots, or control routing.
• Return normal unavailable responses for missing, invalid, unreachable, or hung bridge descriptors, including a bounded status timeout.

Validation

• cargo test -p codex-app-server-protocol --test schema_fixtures --no-fail-fast
• cargo test -p codex-app-server-protocol code_bridge_status_read_is_marked_experimental --no-fail-fast
• cargo test -p codex-app-server-protocol client_request_serialization_scope --no-fail-fast
• cargo test -p codex-code-bridge-client --no-fail-fast
• cargo test -p codex-app-server code_bridge --no-fail-fast
• cargo check -p codex-app-server -p codex-app-server-protocol -p codex-code-bridge-client -p codex-code-bridge-service

Review

• Initial review agents found schema leakage, timeout, and TS optional/null contract issues; fixed all three.
• Post-fix review agents reported no remaining substantive issues.

Refs #50