Show provider execution modes in monitor

[biuworks]

What changed

• add an optional execution mode to monitor events, active/recent requests, and session summaries
• show a responsive Mode column in wide TUI tables, using - for the default mode
• display Codex priority as FAST, Codex flex as FLEX, and Cursor fast execution as FAST
• keep routing prefixes and -fast mode markers out of the resolved model display
• report Cursor FAST mode consistently from Messages, Responses, and Chat Completions paths
• let newer default-mode requests clear a stale session-level FAST value

Why

The monitor previously mixed execution mode markers into the model name, so entries such as composer-2.5-fast appeared as model rewrites and did not clearly show which provider mode was requested. Mode support also differs by provider, so it needs to be optional and provider-reported.

The follow-up fixes address two aggregation and compatibility gaps:

• session aggregation treated None as "keep the old mode" instead of the default mode
• Cursor's OpenAI-compatible generation path used the model-only monitor event

Impact

Wide monitor views now show model and execution mode separately. Normal requests display -, and narrow layouts remain unchanged. Codex and Cursor requests expose their requested mode without requiring every provider to implement a FAST equivalent.

Validation

• cargo test --all-targets — 819 tests passed
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• live Codex and Cursor Messages requests verified FAST-to-default session transitions
• live Codex and Cursor Responses/Chat Completions requests returned HTTP 200 and displayed the expected Mode