Handle logout confirmation read errors

[GreyforgeLabs]

Summary

• Parse logout confirmation through a fallible helper instead of ignoring stdin read errors.
• Treat confirmation read failures as logout failures, so credentials are not removed without confirmed user intent.
• Add regression coverage for yes/no parsing and read-error propagation.

Fixes PlatformNetwork/bounty-challenge#52531.

Validation

• cargo test -p cortex-cli --test logout_confirmation --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check
• git diff --cached --check

[greptile-apps]

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