Refactor execpolicy fallback evaluation (#7544)

## Refactor of the `execpolicy` crate

To illustrate why we need this refactor, consider an agent attempting to
run `apple | rm -rf ./`. Suppose `apple` is allowed by `execpolicy`.
Before this PR, `execpolicy` would consider `apple` and `pear` and only
render one rule match: `Allow`. We would skip any heuristics checks on
`rm -rf ./` and immediately approve `apple | rm -rf ./` to run.

To fix this, we now thread a `fallback` evaluation function into
`execpolicy` that runs when no `execpolicy` rules match a given command.
In our example, we would run `fallback` on `rm -rf ./` and prevent
`apple | rm -rf ./` from being run without approval.

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -180,7 +180,7 @@ async fn run_codex_tool_session_inner(
                         call_id,
                         reason: _,
                         risk,
-                        allow_prefix: _,
+                        proposed_execpolicy_amendment: _,
                         parsed_cmd,
                     }) => {
                         handle_exec_approval_request(