[codex] Improve built-in tool schema docs

[jif-oai]

Summary

• Clarify default, omission, and bounded behavior across built-in tool schemas, including unified exec, classic shell, Code Mode exec/wait, multi-agent, agent job, MCP resource, image, goal, plan, tool_search, and test-sync fields.
• Convert update_plan status to an enum and add short field descriptions where the schema previously relied on surrounding context.
• Remove the dedicated permission-approval schema test and keep only updates to existing expected-spec tests.

Validation

• Ran just fmt.
• Ran git diff --check.
• Did not run clippy or tests, per request.

Regression has been eval here and we proved there are no regressions