[Improvement] Enhance test error messages in CLI tests

Summary

The test code in codex-rs/cli/src/main.rs uses generic panic! messages that could be improved to provide better debugging information when tests fail.

Problem Description

Currently, several tests in the CLI module use generic panic messages like:

• "expected exec subcommand"
• "expected exec resume"
• "expected features enable"

When these tests fail, the panic message doesn't provide enough context about what was actually received, making debugging more difficult.

Location: codex-rs/cli/src/main.rs

• Lines 1099, 1102, 1390, 1393, 1403, 1406

Example

// Current implementation
let Some(Subcommand::Exec(exec)) = cli.subcommand else {
    panic!("expected exec subcommand");
};

// Suggested improvement
let Some(Subcommand::Exec(exec)) = cli.subcommand else {
    panic!("expected exec subcommand, got: {:?}", cli.subcommand);
};

Suggested Improvement

Include the actual value in the panic message to provide debugging context:

panic!("expected exec subcommand, got: {:?}", cli.subcommand);

Or even better, use unreachable!() since these paths should theoretically never be reached (the else branch on a Some pattern match that should always succeed):

let Some(Subcommand::Exec(exec)) = cli.subcommand else {
    unreachable!("expected exec subcommand, got: {:?}", cli.subcommand);
};

Impact

• Priority: Low
• Severity: Quality of life improvement
• Risk: Very low - only affects test code

Benefits

• Faster debugging when tests fail
• Clearer error messages in CI/CD logs
• Better developer experience for contributors

Additional Context

This is a minor code quality improvement that would help with test maintenance and debugging. The tests themselves are correct, only the error messages need enhancement.

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Note: I'm aware that external PRs are by invitation only. I'm submitting this issue to contribute analysis and suggestions as outlined in the contributing guidelines.