Add typed Code Bridge client helpers

[shiny-code-bot]

Summary

• Add typed codex-code-bridge-client helpers for error, pageview, screenshot, and control-result event publishing.
• Add typed screenshot/control request and response helper structs that fill requester/responding client IDs from the active session.
• Rewrite the existing screenshot/control round-trip witness to use the typed helpers, and add event-helper plus capability-denial coverage.

Validation

• cargo fmt --manifest-path codex-rs/Cargo.toml --package codex-code-bridge-client -- --check
• cargo test --manifest-path codex-rs/Cargo.toml -p codex-code-bridge-client
• Read-only review agent: no issues found.

Refs #36