Add Code Bridge browser fixture witness

[shiny-code-bot]

Summary

• add a local Code Bridge browser-app protocol fixture witness using SourceKind::BrowserApp
• exercise browser-shaped page/console events, screenshot request/response with a decoded nonblank PNG, screenshot event delivery, and JavaScript control request/response
• wire --include-code-bridge-witness into the local agent session contract script so this fixture can run alongside the existing contract checks

This is intentionally a browser-app fixture witness, not full DOM/browser automation. Live browser/UI automation for #36 remains a later slice.

Refs #36

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
• python3 scripts/local/e2e_agent_session_contract.py --skip-rust-test --include-code-bridge-witness
• Read-only review agent flagged overclaim risk; the test/script wording was tightened before commit.