Ignore whitespace-only auth environment tokens

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#47006.

Summary

• Add a shared has_non_blank_token() helper in cortex-login.
• Treat empty and whitespace-only auth environment values as absent in whoami, login status, and the engine auth paths.
• Preserve existing token strings when they contain any non-whitespace character.

Validation

• cargo +1.90.0 fmt --package cortex-login --package cortex-engine --package cortex-cli --check
• git diff --check
• Isolated rustc +1.90.0 --edition=2024 --test harness covering empty, whitespace-only, and non-blank tokens

Note: targeted cargo test -p cortex-login test_has_non_blank_token -- --nocapture is still blocked before running tests on this Windows/MSVC environment by the existing aws-lc-sys C build failure.

[greptile-apps]

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