fix: validate logs level filter

[Bortlesboat]

Summary

• Validates cortex logs --level against the supported log levels: error, warn, info, debug, and trace.
• Normalizes accepted levels to lowercase so mixed-case valid input still uses the existing filtering path.
• Adds a focused integration regression test for valid and invalid log-level parsing.

Context

This addresses the validated Platform Bounty Challenge report:

• [BUG] [v0.0.7] cortex logs --level accepts arbitrary strings and matches log content instead of validating log levels PlatformNetwork/bounty-challenge#53518

Verification

• RED: cargo test -p cortex-cli --test logs_level failed because --level rr was accepted.
• GREEN: cargo test -p cortex-cli --test logs_level passed (2 passed).
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

Note: the full cortex-cli lib-test target is currently blocked on main by an unrelated private import in src/cortex-cli/src/agent_cmd/tests.rs; this PR keeps the regression in an integration test so it can run independently.

[greptile-apps]

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