fix(logs): validate level filter values

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53518.

Summary

• Make cortex logs --level a validated clap value instead of accepting arbitrary strings.
• Filter log output by parsed severity tokens rather than raw message substring matches.
• Add regression coverage for invalid level rejection, the warning alias, structured level matching, and message-substring false positives.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check

Targeted cargo +1.90.0 test -p cortex-cli test_logs_cli_level_filter -- --nocapture is blocked in this Windows/MSVC environment before crate tests run by the existing aws-lc-sys C build failure.

[greptile-apps]

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